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dimensions of particle physics
Updated: 31 min 34 sec ago

Howie Day records love song to physics

2 hours 56 min ago

After the musician learned that grad students at CERN had created a parody of his 2004 single “Collide,” he flew to Switzerland to sing it at the LHC.

Singer-songwriter Howie Day was sitting in a coffee shop in Denver one morning while on tour when he saw the Twitter notifications: CERN had shared a parody video of his hit song “Collide,” sung from the perspective of a proton in the Large Hadron Collider.

Sarah Charley, US communications manager for the LHC experiments, had come up with the idea for the video. She created it with the help of graduate students Jesse Heilman of the University of California, Riverside and Tom Perry and Laser Seymour Kaplan of the University of Wisconsin, Madison.

They spent lunches and coffee breaks workshopping their new version of the lyrics, which were originally about two people falling in love despite their differences. They spent a combined 20 hours in CERN’s editing studio recording the vocals and instrumentation of the track. Then they wandered around the laboratory for a full Saturday, filming at various sites. Charley edited the footage together.

“I was flattered, and it was quite funny, too,” Day says of seeing the video for the first time. “I immediately retweeted it and then sent a direct message inquiring about a visit. I figured it was a long shot, but why not?”

That started a conversation that led to Day planning a visit to CERN and booking time in his studio to re-record the song from the ground up with the new lyrics. “It was about the most fun I've ever had in the studio,” Day says. “We literally laughed all day long. I sent the track off to CERN with the note, ‘Should we make another music video?’”

The answer was yes.

While at CERN, Day spent two days visiting the ATLAS and CMS experiments, the CERN Data Centre and the SM18 magnet-testing facility. He also was given the rare opportunity to travel down into the LHC tunnel. CERN’s video crew tagged along to film him at the various sites.

“Going down into the LHC tunnel was a once in a lifetime opportunity, and it felt that way. It was like seeing the northern lights, or playing the Tonight Show, or bringing a new puppy home.”

Day, who says he has always been fascinated by the “why” of things, had been aware of CERN before this project, but he had only a rough idea of what went on there. He says that it wasn’t until he got there that things started to make sense.

“Obviously nothing can prepare you for the sheer scale of the place, but also the people who worked there were amazing,” Day says. “I felt completely overwhelmed and humbled the entire time. It was truly great to be working at the site where humans may make the most important scientific discoveries of our lifetime.”

Heilman, now a postdoctoral researcher at Carleton University, says that he saw the song as a way to reach out to people outside the culture of academia.

“All of us have been steeped in the science for so long that we sort of forget how to speak a language,” he says. “It's always important for academics and researchers to learn different ways to communicate what we’re doing because we’re doing it for people and for society.”

There’s a point in the original song where there’s an emotional build, he says, and Day sings, “I’ve found I’m scared to know, I’m always on your mind.”

The parody uses that part of the song to express the hopes and fears of experimentalists looking for evidence that might not ever appear.

“We're all experimentalists, so we will all spend our careers searching for something,” Heilman says. “The feeling is that [the theory of] supersymmetry, while it's this thing that everybody's been so excited about for a long time, really doesn’t seem that likely to a lot of us anymore because we’re eliminating a lot of the phase space. It's sort of like this white whale hunt. And so our lyrics, ‘Can SUSY still be found?’ is this emotional cry to the physics.”

Charley says she hopes that, through the video, they’re able to “reach and touch people with the science who we normally can't talk to.”

“I think you can appreciate something without fully understanding it,” she says. “As someone who is a professional science communicator, that's always the line I'm walking: trying to find ways that people can appreciate and understand and value something without needing to get a PhD. You can't devote your life to everything, but you can still have an appreciation for things in the world outside your own specific field.”

African School works to develop local expertise

Thu, 06/22/2017 - 16:40

Universities in sub-Saharan Africa are teaming up to offer free training to students interested in fundamental physics.

Last Feremenga was born in a small town in Zimbabwe. As a high school student in a specialized school in the capital, Harare, he was drawn to the study of physics.

“Physics was at the top of my list of potential academic fields to pursue,” he says.

But with limited opportunities nearby, that was going to require a lot of travel.

With help from the US Education Assistance Center at the American Embassy in Harare, Feremenga was accepted at the University of Chicago in 2007. As an undergraduate, he conducted research for a year at the nearby US Department of Energy’s Fermi National Accelerator Laboratory.

Then, through the University of Texas at Arlington, he became one of just a handful of African nationals to conduct research as a user at European research center CERN. Feremenga joined the ATLAS experiment at the Large Hadron Collider. He spent his grad-school years traveling between CERN and Argonne National Laboratory near Chicago, analyzing hundreds of terabytes of ATLAS data.

“I became interested in solving problems across diverse disciplines, not just physics,” he says.

“At CERN and Argonne, I assisted in developing a system that filters interesting events from large data-sets. I also analyzed these large datasets to find interesting physics patterns.”

The African School of Fundamental Physics and Applications

In December 2016, he received his PhD. In February 2017, he accepted a job at technology firm Digital Reasoning in Nashville, Tennessee.

To pursue particle physics, Feremenga needed to spend the entirety of his higher education outside Zimbabwe. Only one activity brought him even within the same continent as his home: the African School of Fundamental Physics and Applications. Feremenga attended the school in the program’s inaugural year at South Africa’s Stellenbosch University.

The ASP received funding for a year from France’s Centre National de la Recherche Scientific (CNRS) in 2008. Since then, major supporters among 20 funding institutions have included the International Center for Theoretical Physics (ICTP) in Trieste, Italy; the South African National Research Foundation, and department of Science and Technology; and the South African Institute of Physics. Other major supporters have included CERN, the US National Science Foundation and the University of Rwanda.

The free, three-week ASP has been held bi-annually since 2010. Targeting students in sub-Saharan Africa, the school has been held in South Africa, Ghana, Senegal and Rwanda. The 2018 School is slated to take place in Namibia. Thanks to outreach efforts, applications have risen from 125 in 2010 to 439 in 2016.

The African School of Fundamental Physics and Applications

The free, three-week ASP has been held bi-annually since 2010. Targeting students in sub-Saharan Africa, the school has been held in South Africa, Ghana, Senegal and Rwanda. The 2018 School is slated to take place in Namibia. Thanks to outreach efforts, applications have risen from 125 in 2010 to 439 in 2016.

The 50 to 80 students selected for the school must have a minimum of a 3-year university education in math, physics, engineering and/or computer science. The first week of the school focuses on theoretical physics; the second week, experimental physics; the third week, physics applications and high-performance computing.

School organizers stay in touch to support alumni in pursuing higher education, says organizer Ketevi Assamagan. “We maintain contact with the students and help them as much as we can,” Assamagan says. “ASP alumni are pursuing higher education in Africa, Asia, Europe and the US.”

Assamagan, originally from Togo but now a US citizen, worked on the Higgs hunt with the ATLAS experiment. He is currently at Brookhaven National Lab in New York, which supports him devoting 10 percent of his time to the ASP.

While sub-Saharan countries are just beginning to close the gap in physics, there is one well-established accelerator complex in South Africa, operated by the iThemba LABS of Cape Town and Johannesburg. The 30-year-old Separated-Sector Cyclotron, which primarily produces particle beams for nuclear research and for training at the postdoc level, is the largest accelerator of its kind in the southern hemisphere.

Jonathan Dorfan, former Director of SLAC National Accelerator Laboratory and a native of South Africa, attended University of Cape Town. Dorfan recalls that after his Bachelor’s and Master’s degrees, the best PhD opportunities were in the US or Britain. He says he’s hopeful that that outlook could one day change.

Organizers of the African School of Fundamental Physics and Applications continue reaching out to students on the continent in the hopes that one day, someone like Feremenga won’t have to travel across the world to pursue particle physics.

A speed trap for dark matter, revisited

Tue, 06/20/2017 - 15:00

A NASA rocket experiment could use the Doppler effect to look for signs of dark matter in mysterious X-ray emissions from space.

Researchers who hoped to look for signs of dark matter particles in data from the Japanese ASTRO-H/Hitomi satellite suffered a setback last year when the satellite malfunctioned and died just a month after launch.

Now the idea may get a second chance.

In a new paper, published in Physical Review D, scientists from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory, suggest that their novel search method could work just as well with the future NASA-funded Micro-X rocket experiment—an X-ray space telescope attached to a research rocket.

The search method looks for a difference in the Doppler shifts produced by movements of dark matter and regular matter, says Devon Powell, a graduate student at KIPAC and lead author on the paper with co-authors Ranjan Laha, Kenny Ng and Tom Abel.

The Doppler effect is a shift in the frequency of sound or light as its source moves toward or away from an observer. The rising and falling pitch of a passing train whistle is a familiar example, and the radar guns that cops use to catch speeders also work on this principle.

This dark matter search technique, called Dark Matter Velocity Spectroscopy, is like setting up a speed trap to “catch” dark matter.

“We think that dark matter has zero averaged velocity, while our solar system is moving,” says Laha, who is a postdoc at KIPAC.  “Due to this relative motion, the dark matter signal would experience a Doppler shift. However, it would be completely different than the Doppler shifts from signals coming from astrophysical objects because those objects typically co-rotate around the center of the galaxy with the sun, and dark matter doesn’t. This means we should be able to distinguish the Doppler signatures from dark and regular matter.”

Researchers would look for subtle frequency shifts in measurements of a mysterious X-ray emission. This 3500-electronvolt (3.5 keV) emission line, observed in data from the European XMM-Newton spacecraft and NASA’s Chandra X-ray Observatory, is hard to explain with known astrophysical processes. Some say it could be a sign of hypothetical dark matter particles called sterile neutrinos decaying in space.

“The challenge is to find out whether the X-ray line is due to dark matter or other astrophysical sources,” Powell says. “We’re looking for ways to tell the difference.”

The idea for this approach is not new: Laha and others described the method in a research paper last year, in which they suggested using X-ray data from Hitomi to do the Doppler shift comparison. Although the spacecraft sent some data home before it disintegrated, it did not see any sign of the 3.5-keV signal, casting doubt on the interpretation that it might be produced by the decay of dark matter particles. The Dark Matter Velocity Spectroscopy method was never applied, and the issue was never settled.  

In the future Micro-X experiment, a rocket will catapult a small telescope above Earth’s atmosphere for about five minutes to collect X-ray signals from a specific direction in the sky. The experiment will then parachute back to the ground to be recovered. The researchers hope that Micro-X will do several flights to set up a speed trap for dark matter.

Jeremy Stoller, NASA

“We expect the energy shifts of dark matter signals to be very small because our solar system moves relatively slowly,” Laha says. “That’s why we need cutting-edge instruments with superb energy resolution. Our study shows that Dark Matter Velocity Spectroscopy could be successfully done with Micro-X, and we propose six different pointing directions away from the center of the Milky Way.”

Esra Bulbul from the MIT Kavli Institute for Astrophysics and Space Research, who wasn’t involved in the study, says, “In the absence of Hitomi observations, the technique outlined for Micro-X provides a promising alternative for testing the dark matter origin of the 3.5-keV line.” But Bulbul, who was the lead author of the paper that first reported the mystery X-ray signal in superimposed data of 73 galaxy clusters, also points out that the Micro-X analysis would be limited to our own galaxy.

The feasibility study for Micro-X is more detailed than the prior analysis for Hitomi. “The earlier paper used certain approximations—for instance, that the dark matter halos of galaxies are spherical, which we know isn’t true,” Powell says. “This time we ran computer simulations without this approximation and predicted very precisely what Micro-X would actually see.”

The authors say their method is not restricted to the 3.5-keV line and can be applied to any sharp signal potentially associated with dark matter. They hope that Micro-X will do the first practice test. Their wish might soon come true.

“We really like the idea presented in the paper,” says Enectali Figueroa-Feliciano, the principal investigator for Micro-X at Northwestern University, who was not involved in the study. “We would look at the center of the Milky Way first, where dark matter is most concentrated. If we saw an unidentified line and it were strong enough, looking for Doppler shifts away from the center would be the next step.”  

From the cornfield to the cosmos

Thu, 06/15/2017 - 23:15

Fermilab celebrates 50 years of discovery.

Imagine how it must have felt to be Robert Wilson in the spring of 1967. The Atomic Energy Commission had hired him as the founding director of the planned National Accelerator Laboratory. Before him was the opportunity to build the most powerful particle accelerator in the world—and to create a great new American laboratory dedicated to giving scientists extraordinary new capabilities to explore the universe. 

Fifty years later, we marvel at the boldness and scope of the project, and at the freedom, the leadership, the confidence and the vision that it took to conceive and build it. If anyone was up for the challenge, it was Wilson. 

By the early 1960s, the science of particle physics had outgrown its birthplace in university laboratories. The accelerators and detectors for advancing research had grown too big, complex and costly for any university to build and operate alone. Particle physics required a new model: national laboratories where the resources of the federal government would bring together the intellectual, scientific, engineering, technical and management capabilities to give collaborations of scientists the ability to explore scientific questions that could no longer be addressed at individual universities. 

The NAL, later renamed Fermi National Accelerator Laboratory, would be a national facility where university physicists—“users”—would be “at home and loved,” in the words of physicist Leon Lederman, who eventually succeeded Wilson as Fermilab director. The NAL would be a truly national laboratory rising from the cornfields west of Chicago, open to scientists from across the country and around the world. 

The Manhattan Project in the 1940s had shown the young Wilson—had shown the entire nation—what teams of physicists and engineers could achieve when, with the federal government’s support, they devoted their energy and capability to a common goal. Now, Wilson could use his skills as an accelerator designer and builder, along with his ability to lead and inspire others, to beat the sword of his Manhattan Project experience into the plowshare of a laboratory devoted to peacetime physics research.  

When the Atomic Energy Commission chose Wilson as NAL’s director, they may have been unaware that they had hired not only a gifted accelerator physicist but also a sculptor, an architect, an environmentalist, a penny-pincher (that they would have liked), an iconoclast, an advocate for human rights, a Wyoming cowboy and a visionary. 

Over the dozen years of his tenure Wilson would not only oversee the construction of the world’s most powerful particle accelerator, on time and under budget, and set the stage for the next generation of accelerators. He would also shape the laboratory with a vision that included erecting a high-rise building inspired by a French cathedral, painting other buildings to look like children’s building blocks, restoring a tall-grass prairie, fostering a herd of bison, designing an 847-seat auditorium (a venue for culture in the outskirts of Chicago), and adorning the site with sculptures he created himself. 

Fermilab physicist Roger Dixon tells of a student who worked for him in the lab’s early days.

“One night,” Dixon remembers, “I had Chris working overtime in a basement machine shop. He noticed someone across the way grinding and welding. When the guy tipped back his helmet to examine his work, Chris walked over and asked, ‘What’ve they got you doin’ in here tonight?’ The man said that he was working on a sculpture to go into the reflecting pond in front of the high rise. ‘Boy,’ Chris said, ‘they can think of more ways for you to waste your time around here, can’t they?’ To which Robert Wilson, welder, sculptor and laboratory director, responded with remarks Chris will never forget on the relationship of science, technology and art.”

Wilson believed a great physics laboratory should look beautiful. “It seemed to me,” he wrote, “that the conditions of its being a beautiful laboratory were the same conditions as its being a successful laboratory.”

With the passage of years, Wilson’s outsize personality and gift for eloquence have given his role in Fermilab’s genesis a near-mythic stature. In reality, of course, he had help. He used his genius for bringing together the right people with the right skills and knowledge at the right time to recruit and inspire scientists, engineers, technicians, administrators (and an artist) not only to build the laboratory but also to stick around and operate it. Later, these Fermilab pioneers recalled the laboratory’s early days as a golden age, when they worked all hours of the day and night and everyone felt like family. 

By 1972, the Main Ring of the laboratory’s accelerator complex was sending protons to the first university users, and experiments proliferated in the laboratory’s particle beams. In July 1977, Experiment E-288, a collaboration Lederman led, discovered the bottom quark. 

Physicist Patty McBride, who heads Fermilab’s Particle Physics Division, came to Fermilab in 1979 as a Yale graduate student. McBride’s most vivid memory of her early days at the laboratory is meeting people with a wide variety of life experiences. 

“True, there were almost no women,” she says. “But out in this lab on the prairie were people from far more diverse backgrounds than I had ever encountered before. Some, including many of the skilled technicians, had returned from serving in the Vietnam War. Most of the administrative staff were at least bilingual. We always had Russian colleagues; in fact the first Fermilab experiment, E-36, at the height of the Cold War, was a collaboration between Russian and American physicists. I worked with a couple of guest scientists who came to Fermilab from China. They were part of a group who were preparing to build a new accelerator at the Institute of High Energy Physics there.” 

The diversity McBride found was another manifestation of Wilson’s concept of a great laboratory.

“Prejudice has no place in the pursuit of knowledge,” he wrote. “In any conflict between technical expediency and human rights, we shall stand firmly on the side of human rights. Our support of the rights of the members of minority groups in our laboratory and its environs is inextricably intertwined with our goal of creating a new center of technical and scientific excellence.”

Designing the future

Advances in particle physics depend on parallel advances in accelerator technology. Part of an accelerator laboratory’s charge is to develop better accelerators—at least that’s how Wilson saw it. With the Main Ring delivering beam, it was time to turn to the next challenge. This time, he had a working laboratory to help.  

The designers of Fermilab’s first accelerator had hoped to use superconducting magnets for the Main Ring, but they soon realized that in 1967 it was not yet technically feasible. Nevertheless, they left room in the Main Ring tunnel for a next-generation accelerator. 

Wilson applied his teambuilding gifts to developing this new machine, christened the Energy Doubler (and later renamed the Tevatron). 

In 1972, he brought together an informal working group of metallurgists, magnet builders, materials scientists, physicists and engineers to begin investigating superconductivity, with the goal of putting this exotic phenomenon to work in accelerator magnets. 

No one had more to do with the success of the superconducting magnets than Fermilab physicist Alvin Tollestrup. Times were different then, he recalls.

“Bob had scraped up enough money from here and there to get started on pursuing the Doubler before it was officially approved,” Tollestrup says. “We had to fight tooth and nail for approval. But in those days, Bob could point the whole machine shop to do what we needed. They could build a model magnet in a week.”

It took a decade of strenuous effort to develop the superconducting wire, the cable configuration, the magnet design and the manufacturing processes to bring the world’s first large-scale superconducting accelerator magnets into production, establishing Fermilab’s leadership in accelerator technology. Those involved say they remember it as an exhilarating experience. 

By March 1983, the Tevatron magnets were installed underneath the Main Ring, and in July the proton beam in the Tevatron reached a world-record energy of 512 billion electronvolts. In 1985, a new Antiproton Source enabled proton-antiproton collisions that further expanded the horizons of the subatomic world. 

Two particle detectors—called the Collider Detector at Fermilab, or CDF, and DZero—gave hundreds of collaborating physicists the means to explore this new scientific territory. Design for CDF began in 1978, construction in 1982, and CDF physicists detected particle collisions in 1985. Fermilab’s current director, Nigel Lockyer, first came to work at Fermilab on CDF in 1984. 

“The sheer ambition of the CDF detector was enough to keep everyone excited,” he says. 

The DZero detector came online in 1992. A primary goal for both experiments was the discovery of the top quark, the heavier partner of the bottom quark and the last undiscovered quark of the six that theory predicted. Both collaborations worked feverishly to be the first to accumulate enough evidence for a discovery. 

In March 1995, CDF and DZero jointly announced that they had found the top. To spread the news, Fermilab communicators tried out a fledgling new medium called the World Wide Web.

Five decades of particle physics 1967

First day NAL employees started work in Illinois


Groundbreaking for the Linac

First NAL Users Meeting


First bison arrive at NAL


Fermilab theorists discover the seeds of superstring theory


Fermilab experimental program begins with experiment E-36 in 100 GeV beam


First tracks in the 15-foot bubble chamber


The National Accelerator Laboratory renamed Fermi National Accelerator Laboratory


Discovery of the bottom quark by the E-288 collaboration

The Department of Energy forms


The High Rise renamed Wilson Hall after founding director Robert Wilson


Tevatron propels protons to 512 GeV, setting world record


Fermilab theorists elucidate the science case for high-energy hadron colliders such as the LHC

Fermilab’s accelerator, previously the Energy Doubler / Saver, is renamed the Tevatron


CDF detector observes first proton-antiproton collisions


Feynman Computing Center dedication


DZero observes first collisions


Discovery of the top quark by CDF and DZero


Sloan Digital Sky Survey sees first light


Evidence of CP violation in neutral B mesons reported by CDF

Fermilab’s KTeV and CERN’s NA48 experiments establish the existence of direct CP violation in kaon decays

Dedication of the Main Injector and Antiproton Recycler accelerators


DONUT collaboration announces direct evidence for tau neutrino


MINOS begins operation in Illinois and Minnesota using new NuMI beam


Discovery of the Higgs boson by the ATLAS and CMS experiments

Dark Energy Survey receives first light


Muon g-2 ring arrives from Brookhaven National Laboratory

NOνA Far Detector records first neutrino


DUNE Collaboration forms


Fermilab’s 50th Anniversary Symposium and 50th Users Meeting

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Meanwhile, in the 1980s, growing recognition of the links between subatomic interactions and cosmology—between the inner space of particle physics and the outer space of astrophysics—led to the formation of the Fermilab Theoretical Astrophysics Group, pioneered by cosmologists Rocky Kolb and Michael Turner. Cosmology’s rapid evolution from theoretical endeavor to experimental science demanded large collaborations and instruments of increasing complexity and scale, beyond the resources of universities—a problem that particle physics knew how to solve. 

In the mid-1990s, the Sloan Digital Sky Survey turned to Fermilab for help. Under the leadership of former Fermilab Director John Peoples, who became SDSS director in 1998, the Sky Survey carried out the largest astronomical survey ever conducted and transformed the science of astrophysics.  

The discovery of cosmological evidence of dark matter and dark energy had profound implications for particle physics, revealing a mysterious new layer to the universe and raising critical scientific questions. What are the particles of dark matter? What is dark energy? In 2004, in recognition of Fermilab’s role in particle astrophysics, the laboratory established the Center for Particle Astrophysics. 

As the twentieth century ended and the twenty-first began, Fermilab’s Tevatron experiments defined the frontier of high-energy physics research. Theory had long predicted the existence of a heavy particle associated with particle mass, the Higgs boson, but no one had yet seen it. In the quest for the Higgs, Fermilab scientists and experimenters made a relentless effort to wring every ounce of performance from accelerator and detectors. 

The Tevatron had reached maximum energy, but in 1999 a new accelerator in the Fermilab complex, the Main Injector, began giving an additional boost to particles before they entered the Tevatron ring, significantly increasing the rate of particle collisions. The experiments continuously re-invented themselves using advances in detector and computing technology to squeeze out every last drop of data. They were under pressure, because the clock was ticking.  

A new accelerator with seven times the Tevatron’s energy was under construction at CERN, the European laboratory for particle physics in Geneva, Switzerland. When Large Hadron Collider operations began, its higher-energy collisions and state-of-the-art detectors would eclipse Fermilab’s experiments and mark the end of the Tevatron’s long run.

In the early 1990s, the Tevatron had survived what many viewed as a near-death experience with the cancellation of the Superconducting Super Collider, planned as a 26-mile ring that would surpass Fermilab’s accelerator, generating beams with 20 times as much energy. Construction began on the SSC’s Texas site in 1991, but in 1993 Congress canceled funding for the multibillion-dollar project. Its demise meant that, for the time being, the high-energy frontier would remain in Illinois. 

While the SSC drama unfolded, in Geneva the construction of the LHC went steadily onward—helped and supported by US physicists and engineers and by US funding. 

Among the more puzzling aspects of particle physics for those outside the field is the simultaneous competition and collaboration of scientists and laboratories. It makes perfect sense to physicists, however, because science is the goal. The pursuit of discovery drives the advancement of technology. Particle physicists have decades of experience in working collaboratively to develop the tools for the next generation of experiments, wherever in the world that takes them. 

Thus, even as the Tevatron experiments threw everything they had into the search for the Higgs, scientists and engineers at Fermilab—literally across the street from the CDF detector—were building advanced components for the CERN accelerator that would ultimately shut the Tevatron down.  

Going global

Just as in the 1960s particle accelerators had outgrown the resources of any university, by the end of the century they had outgrown the resources of any one country to build and operate. Detectors had long been international construction projects; now accelerators were, too, as attested by the superconducting magnets accumulating at Fermilab, ready for shipment to Switzerland.

As the US host for CERN’s CMS experiment, Fermilab built an LHC Remote Operations Center so that the growing number of US collaborating physicists could work on the experiment remotely. In the early morning hours of September 10, 2008, a crowd of observers watched on screens in the ROC as the first particle beam circulated in the LHC. Four years later, the CMS and ATLAS experiments announced the discovery of the Higgs boson. One era had ended, and a new one had begun. 

The future of twenty-first century particle physics, and Fermilab’s future, will unfold in a completely global context. More than half of US particle physicists carry out their research at LHC experiments. Now, the same model of international collaboration will create another pathway to discovery, through the physics of neutrinos. Fermilab is hosting the international Deep Underground Neutrino Experiment, powered by the Long-Baseline Neutrino Facility that will send the world’s most powerful beam of neutrinos through the earth to a detector more than a kilometer underground and 1300 kilometers away in the Sanford Underground Research Facility in South Dakota. 

“We are following the CERN model,” Lockyer says. “We have split the DUNE project into an accelerator facility and an experiment. Seventy-five percent of the facility will be built by the US, and 25 percent by international collaborators. For the experiment, the percentages will be reversed.” 

The DUNE collaboration now comprises more than 950 scientists from 162 institutions in 30 countries. “To design the project,” Lockyer says, “we started with a clean piece of paper and all of our international collaborators and their funding agencies in the room. They have been involved since t=0.”

In Lockyer’s model for Fermilab, the laboratory will keep its historic academic focus, giving scientists the tools to address the most compelling scientific questions. He envisions a diverse science portfolio with a flagship neutrino program and layers of smaller programs, including particle astrophysics. 

At the same time, he says, Fermilab feels mounting pressure to demonstrate value beyond creating knowledge. One potential additional pursuit involves using the laboratory’s unequaled capability in accelerator design and construction to build accelerators for other laboratories. Lockyer says he also sees opportunities to contribute the computing capabilities developed from decades of processing massive amounts of particle physics data to groundbreaking next-generation computing projects. “We have to dig deeper and reach out in new ways.”

In the five decades since Fermilab began, knowledge of the universe has flowered beyond anything we could have imagined in 1967. Particles and forces then unknown have become familiar, like old friends. Whole realms of inner space have opened up to us, and outer space has revealed a new dark universe to explore. Across the globe, collaborators have joined forces to extend our reach into the unknown beyond anything we can achieve separately. 

Times have changed, but Wilson would still recognize his laboratory. As it did then, Fermilab holds the same deep commitment to the science of the universe that brought it into being 50 years ago. 

Fermilab en español (EN)

Tue, 06/13/2017 - 15:00

The particle physics laboratory makes a Spanish connection.

Marylu Reyes and her 12-year-old daughter live just a few miles north of Fermi National Accelerator Laboratory, in West Chicago, Illinois, a town of 27,000 residents with a significant Spanish-speaking population.

When her client, a Fermilab employee, told her the big lab down the street was hosting an event given entirely in Spanish, Reyes and her daughter excitedly marked the date.

What they saw at Fermilab's Pregúntale a un Científico—Ask a Scientist—blew them away.

“When I walked through the lab, it was just like the movies about NASA: big rooms, computers, all that equipment. You felt like you could be a part of it,” says Reyes, who heard presentations on particle accelerators, dark matter and neutrinos. “It was a great opportunity to see it — in our language.”

March’s Pregúntale a un Científico was the first time Fermilab had offered its Ask-a-Scientist, one of the lab’s mainstay public-outreach programs, in Spanish. In fact it was Reyes’ client, Griselda Lopez, who spearheaded the effort. And through the civic engagement of Fermilab’s Hispanic/Latino Forum, a resource group, the successful event, which drew nearly a hundred people, demonstrated the great interest from the surrounding Latino community in the laboratory’s work.   

Pregúntale a un Científico is just one part of Fermilab’s ongoing effort to reach Spanish speakers.

Fermilab is currently developing Spanish-language science materials for the classroom. And it has twice hosted a bilingual conference for a local organization that encourages Latina middle school girls to pursue a STEM education.

“As I was doing these outreach activities, I figured out that it’s not just about science,” said Erika Catano Mur, an Iowa State University graduate student on Fermilab’s NOvA neutrino experiment who has led Spanish-language tours at the lab. “There’s a wall that Spanish-speaking people face that you’re not always aware of. They say, ‘You tell me to go to this website, to call this person to learn more. Do they speak Spanish?’ So we're looking at what’s already out there in Spanish and what more is needed.”

Catano Mur learned English in school in her home country of Colombia, and she speaks English daily at work. Minerba Betancourt, a Fermilab scientist on the MINERvA neutrino experiment who gave presentations at Pregúntale a un Científico, started speaking English regularly only after she came to the United States for graduate school from Venezuela. She continues to speak Spanish with her family.

“I’m proof that you can do science in your second language,” Betancourt says. 

Catano Mur says she rarely does physics in Spanish, since her first language becomes her second language when it comes to physics.

“If I’m talking to another Spanish speaker at the lab, then it can come out in Spanglish, because the science terms come to me much faster in English,” she says.

When talking with nonscientists, Betancourt says, neither language is more difficult than the other. The real translation challenge is moving from jargon into plainspeak. 

It wasn’t just scientists interacting with the attendees at Pregúntale a un Científico. Nontechnical staff were also there to mingle and answer questions.

“We have a rich Spanish-speaking community at the lab—employees, graduate students and postdocs from Latin American and US institutions,” Betancourt says. “Each volunteer contributes something to the wonderful science program at Fermilab.”

The attendees came from all over—not just the surrounding suburbs. Betancourt met one family from Chicago, 40 miles away, and another who lives in Argentina and just happened to be in the area.

When it comes to the lab serving as an educational resource, it is of course nearby residents who have the most to gain, being a stone’s throw away. 

“We have a good community with a great potential for students who could be physicists and engineers,” Betancourt says. “That’s an opportunity I didn’t have — to go to a nearby lab to see what they do.”

It’s as much a chance for the parents as for the children to learn about science careers. 

“The parents are very involved. They sometimes have the idea that if you go into physics, you can be only a high school teacher and have to live a lonely life,” Catano Mur says.”“Any information beyond that is surprising.”

Her goal is to make it less so.

“The Hispanic community here has a big opportunity to get involved in science. A lab like this doesn’t exist in many parts of the world,” Catano Mur says. “A couple of science talks can get the process started.”

Reyes is already well on her way. Even before attending Pregúntale a un Científico, she assumed the role of town crier, distributing flyers about the event at local supermarkets, her daughter’s middle school and her church. It seems to have worked: She saw several friends and acquaintances there.

“I’m so happy that they did this for us. My daughter said, ‘Mom, this was a great experience.’ Reyes says. “I had heard about Fermilab, but I didn’t really know what it was. Now, we feel so welcome.”

(Version en español)

Fermilab en español (ES)

Tue, 06/13/2017 - 15:00

El laboratorio de física de partículas establece una conexión en español.

Marylu Reyes y su hija de 12 años viven a unas pocas millas al norte de Fermi National Accelerator Laboratory, en West Chicago, Illinois, una ciudad de 27,000 habitantes con una población significativa de hispanohablantes.

Cuando la cliente de Reyes, una empleada de Fermilab, le contó que el gran laboratorio del vecindario estaba organizando un evento totalmente en español, Reyes y su hija apuntaron la fecha con gran entusiasmo.

Lo que vieron en Pregúntale a un Científico—Ask a Scientist—de Fermilab las cautivó.

“A medida que recorría el laboratorio, era igual que en las películas sobre la NASA: habitaciones grandes, computadoras, todos esos equipos. Sentías como si pudieras formar parte de ello,” cuenta Reyes, quien escuchó exposiciones sobre aceleradores de partículas, materia oscura y neutrinos. “Fue una gran oportunidad poder presenciarlo… ¡en nuestro idioma!”

Pregúntale a un Científico de marzo fue la primera vez que Fermilab ofreció Ask-a-Scientist, uno de sus principales programas de difusión pública del laboratorio, en idioma español. De hecho, fue la cliente de Reyes, Griselda Lopez, quien encabezó el esfuerzo. Asimismo, a través del compromiso cívico del Foro hispano/latino de Fermilab, un grupo de recursos, el exitoso evento, que atrajo a casi un centenar de personas, demostró el gran interés en el trabajo del laboratorio por parte de la comunidad latina circundante.

Pregúntale a un Científico es solo una parte del esfuerzo continuo de Fermilab para llegar a los hispanohablantes.

En la actualidad, Fermilab se encuentra desarrollando materiales de ciencia en idioma español para el salón de clases. Asimismo, ha organizado en dos oportunidades una conferencia bilingüe para una organización local que alienta a estudiantes latinas de la escuela secundaria a cursar estudios relacionados con la ciencia, la tecnología, la ingeniería y las matemáticas (STEM).

“Mientras estaba realizando estas actividades de difusión, me di cuenta de que no se trata solo de ciencia,” dijo Erika Catano Mur, una estudiante de posgrado de la Universidad Estatal de Iowa (Iowa State University) participante en el experimento NOvA sobre neutrinos de Fermilab, y quien ha guiado recorridos en idioma español dentro del laboratorio. “Existe un muro que enfrentan los hispanohablantes del cual uno no siempre es consciente. Ellos afirman: ‘Me dicen que me dirija a este sitio web para llamar a tal persona a fin de obtener más información. Y esa persona, ¿habla español?’ De modo que estamos observando lo que ya hay disponible en español y qué más se necesita.”

Catano Mur aprendió inglés en la escuela en Colombia, su país natal, y habla dicho idioma a diario en el trabajo. Minerba Betancourt, una científica de Fermilab participante en el experimento MINERvA sobre neutrinos, y quien realizó exposiciones en Pregúntale a un Científico, comenzó a hablar inglés de forma regular solo después de venir a los Estados Unidos desde Venezuela para cursar estudios de posgrado. Ella continúa hablando español con su familia.

“Soy la prueba de que se puede hacer ciencia en tu segundo idioma,” afirmó Betancourt.

Catano Mur dice que rara vez hace física en español. Por lo tanto, su primer idioma se convierte en su segundo idioma cuando se trata de física.

“Si estoy conversando con otro hispanohablante en el laboratorio, entonces podemos hacerlo en Spanglish, porque los términos científicos me vienen a la cabeza mucho más rápido en inglés,” afirma.

Al conversar con no científicos, según Betancourt, ninguno de los idiomas es más difícil que el otro. El verdadero desafío de traducción consiste en pasar los términos técnicos específicos a un léxico sencillo.

No eran solo científicos los que interactuaban con los participantes en Pregúntale a un Científico. Personal no técnico también estaba presente allí para mezclarse y responder preguntas.

“Contamos con una vasta comunidad de hispanohablantes en el laboratorio: empleados, estudiantes de posgrado y posdoctorados de instituciones latinoamericanas y estadounidenses,” contó Betancourt. “Cada voluntario aporta algo al maravilloso programa científico en Fermilab.”

Los participantes acudieron de todas partes, no solo de los suburbios aledaños. Betancourt conoció a una familia de Chicago, que vive a 40 millas de distancia, y otra que vive en Argentina que, casualmente, estaba por la zona.

Cuando se trata del laboratorio como un recurso educativo, los habitantes de los alrededores son, por supuesto, los que tienen más ventajas, ya que se encuentran a pasos del lugar.

“Disponemos de una buena comunidad con un gran potencial de estudiantes que podrían ser físicos e ingenieros,” expresa Betancourt. “Esa es una oportunidad que yo no tuve: ir a un laboratorio cercano para observar lo que hacen.”

Es una oportunidad tanto para padres como para hijos de obtener información sobre carreras científicas.

“Los padres están muy involucrados. A veces tienen la idea de que si te adentras en la física, solo podrás ser profesor de secundaria y tendrás que llevar una vida solitaria,” sostiene Catano Mur. “Cualquier información más allá de eso es sorprendente.”

Su objetivo consiste en reducir eso.

“La comunidad hispana tiene aquí una gran oportunidad de involucrarse en la ciencia. Un laboratorio como este no existe en muchas partes del mundo,” afirma Catano Mur. “Un par de conversaciones científicas puede iniciar el proceso.”

Reyes ya va por buen camino. Incluso antes de asistir a Pregúntale a un Científico, ella asumió el papel de pregonera, distribuyendo volantes acerca del evento en supermercados locales, en la escuela secundaria de su hija y en su iglesia. Parece haber funcionado: Reyes vio a varios amigos y conocidos allí.

“Estoy tan feliz de que hayan hecho esto por nosotros. Mi hija dijo: ‘Mamá, esta fue una gran experiencia,’” contó Reyes. “Había oído acerca de Fermilab pero no sabía realmente qué era. Ahora, nos sentimos muy bien recibidos.”

(English version)

How to clean inside the LHC

Mon, 06/12/2017 - 18:37

The beam pipes of the LHC need to be so clean, even air molecules count as dirt.

The Large Hadron Collider is the world’s most powerful accelerator. Inside, beams of particles sprint 17 miles around in opposite directions through a pair of evacuated beam pipes that intersect at collision points surrounded by giant particle detectors.

The inside of the beam pipes need to be spotless, which is why the LHC is thoroughly cleaned every year before it ramps up its summer operations program.

It’s not dirt or grime that clogs the LHC. Rather, it’s microscopic air molecules.

“The LHC is incredibly cold and under a strong vacuum, but it’s not a perfect vacuum,” says LHC accelerator physicist Giovanni Rumolo. “There’s a tiny number of simple atmospheric gas molecules and even more frozen to the beam pipes’ walls.”

Protons racing around the LHC crash into these floating air molecules, detaching their electrons. The liberated electrons jump after the positively charged protons but quickly crash into the beam pipe walls, depositing heat and liberating even more electrons from the frozen gas molecules there.

This process quickly turns into an avalanche, which weakens the vacuum, heats up the cryogenic system, disrupts the proton beam and dramatically lowers the efficiency and reliability of the LHC.

But the clouds of buzzing electrons inside the beam pipe possess an interesting self-healing feature, Rumolo says.

“When the chamber wall is under intense electron bombardment, the probability of it creating secondary electrons decreases and the avalanche is gradually mitigated,” he says. “Before ramping the LHC up to its full intensity, we run the machine for several days with as many low-energy protons as we can safely manage and intentionally produce electron clouds. The effect is that we have fewer loose electrons during the LHC’s physics runs.”

In other words, accelerator engineers clean the inside of the LHC a little like they would unclog a shower drain. They gradually pump the LHC full of more and more sluggish protons, which act like a scrub brush and knock off the microscopic grime clinging to the inside of the beam pipe. This loose debris is flushed out by the vacuum system. In addition, the bombardment of electrons transforms simple carbon molecules, which are still clinging to the beam pipe’s walls, into an inert and protective coating of graphite.

Cleaning the beam pipe is such an important job that there is a team of experts responsible for it (officially called the “Scrubbing Team”).

“Scrubbing is essential if we want to operate the LHC at its full potential,” Rumolo says. “It’s challenging, because there is a fine line between thoroughly cleaning the machine and accidentally dumping the beam. When we’re scrubbing, we work around the clock in the CERN Control Center to make sure the accelerator is safe and the scrubbing is working properly.”

Another year wiser

Thu, 06/08/2017 - 16:00

In honor of Fermilab’s upcoming 50th birthday, Symmetry presents physics birthday cards.

Some say there are five fundamental interactions: gravitational, electromagnetic, strong, weak and the exchange of birthday greetings on Facebook. But even if you prefer paper to pixels, Symmetry is here to help you celebrate another year. Try these five physics birthday cards, available as both gifs and printable PDFs.

Like two beams of particles in the Large Hadron Collider, your lives intersected. Tell a friend you’re grateful:

Download a printable PDF card Artwork by Corinne Mucha

Like a neutrino, they may change over time, but you still appreciate their friendship:

Download a printable PDF cardArtwork by Corinne Mucha

Whether it's dark energy or another force that pushes them forward, it’s an honor to see them grow: ​

Download a printable PDF cardArtwork by Corinne Mucha

Let them know that, like dark matter, good friends can be hard to find:​

Download a printable PDF cardArtwork by Corinne Mucha

And you’re so glad that, like a long-sought gravitational wave or a Higgs boson, they finally appeared:​

Download a printable PDF cardArtwork by Corinne Mucha

Can’t wait to send your first card? We happen to know of a laboratory with a big day coming up on June 15.

PO Box 500, MS 206
Batavia, IL 60510-5011

(Or reach them on Facebook.)

Print setting recommendations:

Paper Size: Letter
Scale: 100 percent

How to fold your card:

Fold your 8.5 x 11 inch paper in half on the horizontal axis, then fold in half again on the vertical axis. Voilà!

Artwork by Sandbox Studio, Chicago

A tale of three cities

Tue, 06/06/2017 - 15:00

An enormous neutrino detector named ICARUS unites physics labs in Italy, Switzerland, and the US.

Born in Italy, revitalized at CERN and bound for the US, the ICARUS detector is emblematic of modern particle physics experiments: international, collaborative and really, really big.

The ICARUS T600 (if you’re inclined to use the full name) was a pioneer in particle physics technology and is still the largest detector of its kind. When operational, the detector is filled with 760 tons of liquid argon, the same element that, in gas form, makes up about 1 percent of our atmosphere. Since its creation, the ICARUS detector has become a model for modern experiments in the worldwide quest to better understand hard-to-catch particles called neutrinos.

Neutrinos are incredibly small, neutral and rarely interact with other particles, making them difficult to study. Even now, 60 years after their discovery, neutrinos continue to surprise and confound scientists. That’s why this detector with a special talent for neutrino-hunting is undertaking a long journey across the Atlantic to a new home in the United States.

Breaking boundaries at INFN: L’Aquila, Italy

ICARUS got its start in Italy. A groundbreaking large-scale detector, it was the prototype of a sci-fi-sounding instrument called a liquid argon time projection chamber. It functions like four giant cameras, each taking separate 3D images of the signals from neutrinos interacting inside. The active section of the detector is about twice the height of a refrigerator, a couple of meters wider than that and about the length of a bowling lane.

The concept of a liquid argon time projection chamber was proposed in 1977 by physicist Carlo Rubbia, who would later win the Nobel Prize for the discovery of the massive, short-lived subatomic W and Z particles, the carriers of the so-called electroweak force. ICARUS came to life in 2010 at the Gran Sasso National Laboratory, run by Italy’s National Institute for Nuclear Physics (INFN) after decades of development to advance technology and construct the experiment.

At the heart of Gran Sasso Mountain, shielded from cosmic rays raining down from space beneath 1400 meters of rock, it gathered thousands of neutrino interactions during its lifetime. The detector measured neutrinos that traveled 450 miles (730 kilometers) from CERN, but it also saw neutrinos born through natural processes in our sun and our atmosphere. Thus its name: Imaging Cosmic and Rare Underground Signals.

The ICARUS collaboration studied various properties of neutrinos, including a surprising phenomenon called neutrino oscillation. Neutrinos come in three varieties, or flavors, and have the uncommon ability to change from one type to another as they travel. But the proof of technology was just as important as the knowledge the experiment gained about neutrinos. ICARUS showed that liquid argon technology was an efficient, reliable and precise way to study the elusive particles.

“Following its initial conception, the experimental development from a table-top device to the huge ICARUS detector has required a number of successive steps in an experimental journey that has lasted almost 20 years,” says Carlo Rubbia, spokesperson of the ICARUS collaboration. “The liquid argon, although initially coming from air, must reduce impurities to a few parts per trillion, a tiny amount in volume and free electron lifetimes of 20 milliseconds. Many truly remarkable collaborators have participated in Italy in the creation of such a novel technology.”

CERN shut down its neutrino beam in 2012, but ICARUS had more to offer. Scientists decided to move the detector to the US Department of Energy’s Fermi National Accelerator Laboratory, to make use of one of its intense neutrino beams.

To make the transition, ICARUS needed an upgrade. Workers maneuvered ICARUS out of the crowded Gran Sasso lab, packed it in two modules (drained of liquid argon) onto special transporters, and wound their way through the Alps to just the place to get an upgrade, the European particle physics laboratory CERN.

A rebirth at CERN: Geneva, Switzerland

After traversing the Mont Blanc tunnel and winding through small French villages toward Geneva, the two large ICARUS modules arrived at CERN in December 2014. After several years of operation at Gran Sasso, the detector was ready for a reboot. One of the main tasks was updating all the electronics and the read-out system.

“The detector itself is very modern and sophisticated, but the supporting technology has evolved over the last 20 years,” says Andrea Zani, a CERN researcher working on the ICARUS experiment. “For example, the original cables are not produced anymore, and the new data read-out system will be higher-performing, exploiting newer components that are far more compact.”

Zani and his colleagues started disassembling parts of the detector at Gran Sasso and then continued their work at CERN. They are replacing the old electronics with 50,000 new read-out channels, which streamline the data collection process and will improve the experiment’s performance overall. Other upgrades involved realigning components to improve the detector’s precision.

“The high-voltage cathode plates were slightly deformed in a few places, which was fine when the experiment first started operation,” Zani says, “but we now we have the capability to make even more precise measurements. We had to heat and then press the plates until they were almost perfectly flat.”

The team also replaced a few dozen outdated light sensors with 360 new photomultiplier tubes, which are now nested behind the wires lining the inner walls of their detectors.

When neutrinos strike atoms of argon in one of the detectors, they release a flash of light and a cascade of charged particles. As these charged particles pass through the detector they ionize other argon atoms releasing electrons. An electric field across the detector causes these electrons to drift toward a plane of roughly 13,000 wires (52,000 in total, counting all four sections of the detector), which measure the incoming particles and enable scientists to reconstruct finely detailed images.

“In addition to the cascade of ionized particles, neutrinos produce a tiny flash of ultraviolet light when they interact with argon atoms,” Zani says. “We know the velocity of electrons as they travel through the liquid argon, and can calculate a particle’s distance from the wire detectors based on the time it takes for the electrical signal to arrive after this flash.

These precise location measurements help scientists distinguish between interesting neutrino interactions and ordinary cosmic rays. Before their installation, all 360 new photomultiplier tubes had to be dusted with a fine powder that shifts the original UV light into a deep blue glow. Over the course of several months, a dedicated team of physicists and technicians completed the process of dusting, testing and finally installing the new light sensors.

In addition to refurbishing the detector, CERN’s engineering team designed and built two huge coolers that will eventually hold the two large ICARUS modules. These containers work much like a thermos and use a vacuum between their inner and outer walls. A layer of solid foam between them will prevent heat from seeping into the experiment. An international collaboration of scientists and engineers are also developing the supporting infrastructure that will enable ICARUS to integrate into its new home at Fermilab.

The final step was stress-testing the containers and packaging the detector for its long journey across the Atlantic.

“It’s been a lot of work,” Zani says, “and putting this all together has been a close collaboration between many different institutions. But we all have the common goal of preparing this detector for its second life at Fermilab.”

New horizons at Fermilab: Batavia, Illinois

While the ICARUS detector was getting an upgrade at CERN, teams of people at Fermilab were preparing for its arrival.

In July 2015, work began on the building that will house the detector 30 feet underground, precisely in the path of Fermilab’s neutrino beam. To keep the cryogenic vessels cold, a team of workers from CERN and INFN visited Fermilab in May 2017 to help install a steel structure that will hold a hefty amount of insulation.

“We couldn’t do this without our partners around the world, and it’s been very rewarding to see it all come together,” says Peter Wilson, the head of Fermilab’s short-baseline neutrino program. “The steel vessel was designed by CERN and manufactured in Poland. The electronic systems were designed by INFN. We’re working with CERN, INFN and other institutions on cosmic-ray taggers that will go above, around and below the detector.”

When the ICARUS detector arrives, it will spend a couple of months undergoing tests and final preparations before being lowered by crane into the building. Once there, it will take its place as the largest in a suite of three detectors on site at Fermilab with a common purpose: to search for a theorized, but never seen, fourth type of neutrino.

Scientists have observed three types of neutrinos: the muon, the electron and the tau neutrino. But they have also seen hints that those three types might be changing into another type they can’t detect. Two experiments in particular—the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Lab and MiniBooNE at Fermilab—saw an unexplained excess of charged particles of unexplained origin. One theory is that they were produced by so-called “sterile” neutrinos, which would not interact in the same way as the other three neutrinos.

ICARUS will join the Short-Baseline Near Detector, currently under construction, and MiniBooNE’s successor, MicroBooNE, which has been taking data for nearly two years, on the hunt for sterile neutrinos. All three detectors use the same liquid-argon technology pioneered for ICARUS.

The journey of the ICARUS detector could have a destination beyond its new home at Fermilab. If evidence of a new kind of neutrino were discovered, it could travel all the way to a new understanding of the universe.  

Muon magnet’s moment has arrived

Thu, 06/01/2017 - 18:19

The Muon g-2 experiment has begun its search for phantom particles with its well-traveled electromagnet.

What do you get when you revive a beautiful 20-year-old physics machine, carefully transport it 3200 miles over land and sea to its new home, and then use it to probe strange happenings in a magnetic field? Hopefully you get new insights into the elementary particles that make up everything.

The Muon g-2 experiment, located at the US Department of Energy’s Fermi National Accelerator Laboratory, has begun its quest for those insights.

Take a 360-degree tour of the Muon g-2 experimental hall.

On May 31, the 50-foot-wide superconducting electromagnet at the center of the experiment saw its first beam of muon particles from Fermilab’s accelerators, kicking off a three-year effort to measure just what happens to those particles when placed in a stunningly precise magnetic field. The answer could rewrite scientists’ picture of the universe and how it works.

“The Muon g-2 experiment’s first beam truly signals the start of an important new research program at Fermilab, one that uses muon particles to look for rare and fascinating anomalies in nature,” says Fermilab Director Nigel Lockyer. “After years of preparation, I’m excited to see this experiment begin its search in earnest.”

Getting to this point was a long road for Muon g-2, both figuratively and literally. The first generation of this experiment took place at Brookhaven National Laboratory in New York State in the late 1990s and early 2000s. The goal of the experiment was to precisely measure one property of the muon—the particles’ precession, or wobble, in a magnetic field. The final results were surprising, hinting at the presence of previously unknown phantom particles or forces affecting the muon’s properties.

The new experiment at Fermilab will make use of the laboratory’s intense beam of muons to definitively answer the questions the Brookhaven experiment raised. And since it would have cost 10 times more to build a completely new machine at Brookhaven rather than move the magnet to Fermilab, the Muon g-2 team transported that large, fragile superconducting magnet in one piece from Long Island to the suburbs of Chicago in the summer of 2013.

The magnet took a barge south around Florida, up the Tennessee-Tombigbee waterway and the Illinois River, and was then driven on a specially designed truck over three nights to Fermilab. And thanks to a GPS-powered map online, it collected thousands of fans over its journey, making it one of the most well-known electromagnets in the world.

“Getting the magnet here was only half the battle,” says Chris Polly, project manager of the Muon g-2 experiment. “Since it arrived, the team here at Fermilab has been working around the clock installing detectors, building a control room and, for the past year, adjusting the uniformity of the magnetic field, which must be precisely known to an unprecedented level to obtain any new physics. It’s been a lot of work, but we’re ready now to really get started.”

That work has included the creation of a new beamline to deliver a pure beam of muons to the ring, the installation of a host of instrumentation to measure both the magnetic field and the muons as they circulate within it, and a year-long process of “shimming” the magnet, inserting tiny pieces of metal by hand to shape the magnetic field. The field created by the magnet is now three times more uniform than the one it created at Brookhaven. 

Over the next few weeks the Muon g-2 team will test the equipment installed around the magnet, which will be storing and measuring muons for the first time in 16 years. Later this year, they will start taking science-quality data, and if their results confirm the anomaly first seen at Brookhaven, it will mean that the elegant picture of the universe that scientists have been working on for decades is incomplete, and that new particles or forces may be out there, waiting to be discovered.

“It’s an exciting time for the whole team, and for physics,” says David Hertzog of the University of Washington, co-spokesperson of the Muon g-2 collaboration. “The magnet has been working, and working fantastically well. It won’t be long until we have our first results, and a better view through the window that the Brookhaven experiment opened for us.”

Editor's note: This article is based on a Fermilab press release.

At LIGO, three’s a trend

Thu, 06/01/2017 - 17:09

The third detection of gravitational waves from merging black holes provides a new test of the theory of general relativity.

For the third time, the LIGO and Virgo collaborations have announced directly detecting the merger of black holes many times the mass of our sun. In the process, they put general relativity to the test.

On January 4, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory stretched and squeezed ever so slightly, breaking the symmetry between the motions of two sets of laser beams. This barely perceptible shiver, lasting a fraction of a second, was the consequence of a catastrophic event: About 3 billion light-years away, a pair of spinning black holes with a combined mass about 49 times that of our sun sank together into a single entity.

The merger produced more power than is radiated as light by the entire contents of the universe at any given time. “These are the most powerful astronomical events witnessed by human beings,” says Caltech scientist Mike Landry, head of the LIGO Hanford Observatory.

When the black holes merged, about two times the mass of the sun converted into energy and released in the form of ripples in the fabric of existence. These were gravitational waves, predicted by Albert Einstein’s theory of general relativity a century ago and first detected by LIGO in 2015.

“Gravitational waves are distortions in the medium that we live in,” Landry says. “Normally we don’t think of the nothing of space as having any properties of all. It’s counterintuitive to think it could expand or contract or vibrate.”

It was not a given that LIGO would be listening when the signal from the black holes arrived. “The machines don’t run 24-7,” says LIGO research engineer Brian Lantz of Stanford University. The list of distractions that can sabotage the stillness the detectors need includes earthquakes, wind, technical trouble, moving nitrogen tanks, mowing grass, harvesting trees and fires.

When the gravitational waves from the colliding black holes reached Earth in January, the LIGO detectors happened to be coming back online after a holiday break. The system that alerts scientists to possible detections wasn’t even fully back in service yet, but a scientist in Germany was poring over the data anyway.

“He woke us up in the middle of the night,” says MIT scientist David Shoemaker, newly elected spokesperson of the LIGO Scientific Collaboration, a body of more than 1000 scientists who perform LIGO research together with the European-based Virgo Collaboration.

The signal turned out to be worth getting out of bed for. “This clearly establishes a new population of black holes not known before LIGO discovered them,” says LIGO scientist Bangalore Sathyaprakash of Penn State and Cardiff University.

The merging black holes were more than twice as distant as the two pairs that LIGO previously detected, which were located 1.3 and 1.4 billion light-years away. This provided the best test yet of a second prediction of general relativity: gravitons without any mass.

Gravitons are hypothetical particles that would mediate the force of gravity, just as photons mediate the force of electromagnetism. Photons are quanta of light; gravitons would be quanta of gravitational waves.

General relativity predicts that, like photons, gravitons should have no mass, which means they should travel at the speed of light. However, if gravitons did have mass, they would travel at different speeds, depending on their energy.

As merging black holes spiral closer and closer together, they move at a faster and faster pace. If gravitons had no mass, this change would not faze them; they would uniformly obey the same speed limit as they traveled away from the event. But if gravitons did have mass, some of the gravitons produced would travel faster than others. The gravitational waves that arrived at the LIGO detectors would be distorted.

“That would mean general relativity is wrong,” says Stanford University Professor Emeritus Bob Wagoner. “Any one observation can kill a theory.”

LIGO scientists’ observations matched the first scenario, putting a new upper limit on the mass of the graviton—and letting general relativity live another day. “I wouldn’t bet against it, frankly,” Wagoner says.

Like a pair of circling black holes, research at LIGO seems to be picking up speed. Collaboration members continue to make improvements to their detectors. Soon the complementary Virgo detector is expected to come online in Italy, and in 2024 another LIGO detector is scheduled to start up in India. Scientists hope to eventually see new events as often as once per day, accumulating a pool of data with which to make new discoveries about the goings-on of our universe.

A brief etymology of particle physics

Tue, 05/30/2017 - 15:00

How did the proton, photon and other particles get their names?

Over the years, physicists have given names to the smallest constituents of our universe.

This pantheon of particles has grown alongside progress in physics. Anointing a particle with a name is not just convenient; it marks a leap forward in our understanding of the world around us. 

The etymology of particle physics contains a story that connects these sometimes outlandish names to a lineage of scientific thought and experiment.

So, without further ado, Symmetry presents a detailed guide to the etymology of particles—some we’ve found and others we have yet to discover.

Editor’s note: PIE, referenced throughout, refers to proto-Indo-European, one of the earliest known languages.

Discovered particles Expand all ion ion Named by: William Whewell, 1834

Ions are atoms or molecules that are charged. The term “ion” was coined by 19th-century polymath William Whewell, who developed it for his contemporary Michael Faraday, who made important discoveries in the realm of electromagnetism. “Ion" comes from the neuter present participle of Greek ienai, “go,” to describe the particle’s attraction, or tendency to move toward opposite charges. Ienai originates from the PIE ei, “to go, to walk.”

The suffix “-on” derives from “ion” and appears in the names of many particles.

fermion Fermi + on Named by: Paul Dirac, 1945

Fermions (which include the proton and electron) were named for physicist Enrico Fermi. Fermi developed the first statistical formulas that govern fermions, particles that follow the Pauli exclusion principle, which states that certain particles can’t occupy the same quantum space.

lepton leptos + on Named by: Christian Møller and Abraham Pais, 1947

Leptons are a class of particles that includes the electron, muon, tau and neutrinos. The name “lepton” was suggested as a counterpart to the nucleon, a name for the particles that make up the atomic nucleus, according to a biography of Abraham Pais.

The first known lepton, the electron, is much lighter than a nuleon. Hence the root word for lepton: the Greek leptos, meaning “small, slight, slender, delicate, subtle,” which originates from PIE lep, meaning “peel” and “small shaving.” This root is also shared by the word “leprosy,” so named because it is a disease that causes scabbing and weakness.

In 1920, chemists had suggested the name lepton for all electrons, atoms, ions and molecules, but it did not catch on.

electron electric + on Named by: George Stoney, 1891

Electrons are negatively charged leptons that orbit the nucleus of an atom. Late-19th-century physicist George Stoney came up with the term “electron” to describe what he called in a letter “this most remarkable fundamental unit of electricity.”

The word "electric” was first used to describe materials with an attractive force in the early 17th century. “Electric” itself derives from New Latin electricus, which was used in 1600 to characterize the magnetic attraction of amber when it was rubbed. Electricus was taken from Latin electrum, from Greek elektron, both of which refer specifically to amber.

muon mu-meson (contraction) Named by: Carl Anderson and Seth Neddermeyer, 1938

Muons are members of the lepton family and behave like heavier cousins to electrons.

The muon was originally called a “mesotron,” from the Greek word mesos, meaning “middle,” or “intermediate,” according to a letter published in Nature. That’s because its mass was considered to be in the middle, between that of an electron and a proton.

However, more particles with masses between that of electrons and protons were discovered, and meson became a general term to describe them, according to an article in Engineering and Science Monthly. Around 1949 the initial particle was renamed “mu-meson,” corresponding to the Greek letter mu (µ) (see article, subscription required).

Later, scientists discovered differences between the mu-meson and other mesons, which led to the mu-meson being reclassified as a lepton and having its name shortened to just muon.

tau triton Named by: Martin Perl, 1975

Known also as “the tau particle,” “tau lepton” and even “tauon,” this particle became the third charged lepton—after the electron and muon—when it was discovered in 1975. Because of its third-place finish, it was given the symbol tau (τ) for the Greek triton, meaning “third” (see paper). (Why they didn’t just name it a “triton” remains a mystery.)

neutrino neutro (diminutive) Named by: Enrico Fermi, 1933

In 1930, physicist Wolfgang Pauli was studying the problem of energy going missing in a type of particle decay. He proposed that the energy was being carried away by a neutral particle that scientists could not detect. He called this a “neutron,” a combination of the root of the word “neutral”—which derives from Latin neuter meaning “neither gender”—with the suffix “-on.”

However, in 1932, another neutral particle was discovered and also called a “neutron.” This second neutron was heavy and existed in the nucleus. In 1933, physicist Enrico Fermi discovered the original particle Pauli had been describing. To distinguish it from the second neutron, which was more massive, he added to the name the Italian diminutive suffix “-ino.”

Neutrinos come in three flavors that correspond to their charged-lepton cousins: electron, muon and tau.

quark quark Named by: Murray Gell-Mann, 1963

Quarks are elementary particles that form hadrons such as protons and neutrons, as well as more exotic particles and states of matter like quark-gluon plasma. They were proposed simultaneously by Murray Gell-Mann and George Zweig (who wanted to call them “aces”), and different types of quarks were discovered throughout the rest of the 20th century by multiple different teams of physicists.

Gell-Man wrote about the name in his popular science book The Quark and the Jaguar:

In 1963, when I assigned the name “quark” to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been “kwork.” Then, in one of my occasional perusals of Finnegan’s Wake, by James Joyce, I came across the word “quark” in the phrase “Three quarks for Muster Mark”.

Since “quark” (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with “Mark,” as well as “bark” and other such words, I had to find an excuse to pronounce it as “kwork.” But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the “portmanteau” words in Through the Looking-Glass. From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar.

I argued, therefore, that perhaps one of the multiple sources of the cry “Three quarks for Muster Mark” might be “Three quarts for Mister Mark,” in which case the pronunciation “kwork” would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.

Some scholars suspect that the quark in Joyce’s epic derives from the German quark, which is a type of cheese curd. The German quark is likely taken from West Slavic words meaning “to form”—potentially a reference to milk solidifying and becoming curd. Serendipitously, “to form” is also the non-dairy quark’s role as the main constituent of matter.

Physicists have discovered six types of quarks, named “up,” “down,” “strange,” “charm,” “top” and “bottom.”

up and down quarks: Gell-Mann named these quarks in 1964 for their “upward” and “downward” isospin, which is a quantum property of particles related to the strong nuclear force.

strange: Unlike up and down quarks, strange quarks were observed before the quark model was developed, as constituents of composite particles called kaons. These particles were deemed "strange" because they had unusually long lifetimes, due to some of their decays occurring through the weak force. Gell-Man called them “strange quarks” in 1964.

charm: The charm quark was predicted in a paper by two physicists, Sheldon Glashow and James Bjorken, in 1964. As they explained in a New York Times article: “We called our construct the ‘charmed quark,’ for we were fascinated and pleased by the symmetry it brought to the subnuclear world.” “Charm,” meaning “pleasing quality,” is derived from the Latin carmen, “song, verse, enchantment.”

top and bottom: Physicists Makoto Kobayashi and Toshihide Maskawa predicted the existence of the last two quarks in 1973, but they did not assign names to the new particles. Many scientists unofficially called them “truth” and “beauty.”

In a 1975 paper, physicist Haim Harari gave them names that stuck. To preserve the initials “t” and “b” and create a fitting counterpart for up and down quarks, Harari called them “top” and “bottom” quarks.

boson Bose + on Named by: Paul Dirac, 1945

Bosons were named for physicist Satyendra Nath Bose. Along with Albert Einstein, Bose developed a theory explaining this type of particle, which had integer spins and therefore did not obey the Pauli exclusion principle. Because bosons don’t obey the exclusion principle, they can essentially exist on top of one another, or in “superposition.” Bose’s work developing a theory for bosons, a class that include “force carriers” such as photons and gluons, is an integral part of the Standard Model.

photon photo + on Named by: unclear

Photons are sometimes called particles of light. Although the concept of a particle of light (as opposed to a light wave) had been around for over two decades by the time Einstein’s seminal paper on the photoelectric effect was published in 1905, there was still not a widely accepted name for the phenomenon. The term “photon” became accepted in 1927 after Arthur Compton won the Nobel Prize for the discovery of Compton scattering, a phenomenon that demonstrated unquestionably that light was quantized.

The modern origins of the idea of light as a particle date back to 1901. Physicist Max Planck wrote about “packets of energy” as quanta, from the Latin quantum, meaning “how much.”

This was adapted by Albert Einstein, who referred to discrete “wave packets” of light as das Lichtquant or “the light quantum” (see paper, in German).

The first known use of the word “photon” was by physicist and psychologist Leonard Troland, who used it in 1916 to describe a unit of illumination for the retina. Photon derives from the Greek phos, “light,” from PIE bha “to shine.”

Five years later, Irish physicist John Joly used the word to describe the “unit of luminous sensation” created by the cerebral cortex in his effort to create a “quantum theory of vision.”

In 1924, a French biochemist used the word, and in 1926, a French physicist picked it up as well. But the word did not catch on among the physics community until a few months later, when American physical chemist Gilbert Lewis (famous for discovering the covalent bond) began using it.

As described in Progress in Optics, Lewis’ concept of a photon was fundamentally different from Einstein’s—for one, Gilbert incorrectly posited that the number of photons was a conserved quantity. Still, the term finally stuck, and has been used ever since.

Higgs boson Higgs + boson Named by: unclear

The Higgs boson is the particle associated with the field that gives some elementary particles their mass. It is called “the Higgs” in honor of British theorist Peter Higgs, who predicted its existence in 1964.

However, Higgs wasn’t the only theorist to contribute to the theory of the particle. Others credited with its prediction include Robert Brout, Francois Englert, Philip Anderson, Gerald Guralnik, Carl Hagen, Tom Kibble and Gerard t’Hooft.

The particle has also been called “the Bout-Englert-Higgs particle,” “the Anderson-Higgs particle,” or even “the Englert-Brout-Higgs-Guralnik-Hagen-Kibble” or “ABEGHHK’tH particle.”

According to an article in Nature, this extensive list of names was pared down by theorists such as Benjamin Lee, who referred to it as “the Higgs,” and by Steven Weinberg, who (mistakenly) cited Higgs in a paper (subscription required) as having provided the first theory to explain why some particles have mass.

In an effort to drive popular support for the search for the Higgs boson, physicist Leon Lederman gave it the moniker “The God Particle.” For his part, Higgs the theorist often refers to the “scalar boson” or “so-called Higgs particle.”

W boson weak + boson Named by: T.D. Lee and C.N. Yang, 1960

Carriers of the weak nuclear force in charged current interactions, W bosons were first predicted and named in a paper (subscription required) in 1960. W bosons likely draw their name from the weak nuclear force, so called because its field strength over a given distance is much weaker than the strong and electromagnetic forces. The word weak comes from Old Norse veikr “weak” with potential origins tracing back to PIE weik, “to bend, wind.”

Z boson zero + boson Named by: Sheldon Glashow, 1961

Like W bosons, Z bosons are mediators for the weak force. Unlike W bosons, though, Z bosons have no charge, so exchanges of Z bosons are called “neutral current interactions.”

When Sheldon Glashow theorized them in a paper in 1961, he did not provide an explanation. Some theories allege that Z stands for “zero” because of the neutral current’s lack of charge. Zero has its roots in Italian zero, which comes from Medieval Latin zephirum. Italian mathematician Leonardo Fibonacci coined zephirum, meaning “zero,” from Arabic sifr, “nothing.” Sifr is likely a translation of Sanskrit sunya-m, “empty place, desert.”

gluon glue + on Named by: Murray Gell-mann, 1962

Gluons are mediators of the strong force, which is what holds the nucleus together. Interactions through the strong force can be thought of as exchanges of gluons.

Gluons were ostensibly named for their glue-like properties and ability to keep the nucleus together (see paper, subscription required). Glue derives from Early French glu and has its roots in Latin gluten “to glue,” which is also the origin of gluten, the “nitrogenous part of grain.” However, there are no foods that are gluon-free.

hadron hadros + on Named by: Lev Okun, 1962

The term “hadron” was coined at the 1962 International Conference on High Energy Physics (see report) to refer to heavier partner particles to leptons. Hadron comes from the Greek hadros, meaning “thick, bulky, massive.” It was later discovered that hadrons were composite particles made up of quarks surrounded by a haze of gluons.

baryon barys + on Named by: Abraham Pais, 1953

Baryons are a kind of hadron that is made of three quarks held together by gluons. Protons and neutrons, which make up the nucleus of atoms, are both baryons.

The use of the word “baryon” appeared in 1953, when physicist Abraham Pais proposed it as a name for nucleons and other heavy particles. It draws from barys, the Greek word for “heavy.”

proton protos + on Named by: Ernest Rutherford, 1920

The proton is one of the three constituents of an atom, along with neutrons and electrons.

Physicist Ernest Rutherford proposed the name in honor of 19th century scientist William Prout. In 1816 Prout proposed calling the hydrogen atom a “protyle,” from the Greek protos, “first,” and húlē, “material.” Prout believed hydrogen was the constituent atom for all elements.

Prout was later proven wrong, but Rutherford suggested calling the particle he discovered either “proton”—after Prout’s hypothetical particle—or “prouton”—after Prout himself. Rutherford and other scientists eventually settled on proton, whose root was also the Greek protos.

neutron neutral + on Named by: unclear

Neutrons are particles made of up and down quarks. According to a letter published in Nature, it is unclear whether physicist William Harkins or physicist Ernest Rutherford referred to the electrically neutral nucleon as a “neutron” first. What is clear is that both came up with the same name for the same particle in 1921, likely drawing on the same etymology of the root word neutral.

meson mesos + on Named by: Homi J. Bhabha, 1939

Mesons are particles made of both a quark and an anti-quark.

Mesons were originally referred to as “heavy electrons,” as their masses were between the electron and the proton, or as “U-particles” for their unknown nature, or as “Yukawa particles” after physicist Hideki Yukawa, who first theorized them in 1935. In the past, mesons were also used inaccurately to refer to bosons.

Carl Anderson and Seth Neddermeyer, co-discoverers of the muon, suggested calling the particle a “mesotron,” derived from the Greek word mesos, meaning “middle,” for their intermediate masses. Physicist Homi J. Bhabha, considered the father of nuclear physics in India, suggested in an article (subscription required) the shorter name “meson” in 1939.

Many mesons, such as kaons and pions, are simply contractions named after the letters used to represent them (K-meson, Pi-meson).

antimatter anti + matter Named by: unclear

Particles of matter have partner particles of antimatter, which share the same mass, but have opposite electrical charge and spin. When a matter-antimatter pair meets, the particles annihilate one another.

In 1928, theorist Paul Dirac theorized in a paper what he called the “anti-electron,” the first hypothetical particle of antimatter. However, when Carl Anderson discovered the particle in 1932, he called it a “positron” because of its positive charge. (According to an article by Cecilia Jarlskog, an international group of physicists suggested in 1948 that the positron should be called a “positon” and the electron should be renamed a “negaton,” but the effort never quite caught on.)

Around 1937, Dirac’s original “anti-” prefix came back into use to describe particles like the positron (see article, subscription required).

Possibly the first reference to modern antimatter came in 1948 (see article, subscription required). It’s likely that it took so long to come up with a generic term due to the limited number of particles and antiparticles that had been discovered at that time.

The actual first use of the term occurred in 1898 as part of a somewhat whimsical letter published in Nature (subscription required) proposing the existence of matter with “negative gravity.”

The prefix “anti-“ originates from Greek anti, meaning “against, opposed to, opposite of, instead.” The word “matter,” meaning “physical substance,” is a 14th-century construction that comes from materie, “subject of thought, speech, or expression,” itself deriving from Latin material, or “substance from which something is made.” This comes from Latin mater "origin, source, mother.”

Hypothetical particles Expand all axion Axion Named by: Frank Wilczek, 1978

Axions are hypothetical particles and candidates for the dark matter that is thought to potentially make up most of the mass in the universe. Frank Wilczek said in a Nobel lecture that he “named them after a laundry detergent, since they clean up a problem.”

Said problem is known as “the Strong CP problem,” which is an unsolved question of why quark interactions and anti-quark interactions seem to follow the same rules.

chameleon chameleon Named by: Justin Khoury and Amanda Weltman, 2003

The chameleon particle is a hypothetical particle of dark energy.

The word “chameleon” comes from the Greek cognate khamaileon, whose root khamai means “on the ground.” Its other root, leon means lion; thus “ground lion.” But the name chameleon comes from the defining characteristic of lizards of that name. In a 2003 paper, physicists Justin Khoury and Amanda Weltman proposed and named the particle, the physical characteristics of which would depend on its environment.

graviton gravity + on Named by: Dmitri Blochinzew and F. M. Gal’perin, 1934

The graviton, an undiscovered particle associated with the force of gravity, is one of the oldest hypothetical particles (see paper, in Russian). It takes its name from the English “gravity,” which itself comes from Old French gravité meaning “seriousness, thoughtfulness.” The Latin root, gravis “heavy,” was repurposed as gravity for scientific use in the 17th century to mean “weight.”

Perhaps the earliest use of the word comes from the 1644 philosophical text Two Treatises: of Bodies and of Man’s Soul. It would be another 40 odd years until Isaac Newton made gravity mathematically rigorous in his Principia.

majoron Majorana + on Named by: Y. Chikashige, Rabindra Mohapatra, and Roberto Peccei, 1980

In particle physics, “lepton number” is the number of leptons in a particle reaction minus the number of antileptons. As far as we know, lepton number must be conserved from the beginning to the end of an interaction.

A majoron is hypothetical type of boson proposed to solve problems with the conservation of lepton number thought to exist in some high-energy collisions (see paper, subscription required). Majorons were named after Majorana fermions, named after physicist Ettore Majorana, who hypothesized the existence of particles that were their own antiparticles. Majorana, a variant of Maiorana, an Italian surname popular in Sicily, owes its roots to the herb marjoram, which is common in that area.

tachyon tachy + on Named by: Gerald Feinberg, 1967

Proposed in a 1967 paper (subscription required) as a name for hypothetical faster-than-light particles, tachyons take their name from the Greek takhys for “swift.”

supersymmetric particles super + symmetry Named by: Abdus Salam, J. Strathdee, 1974

Supersymmetry is a theory that about doubles the number of particles in the Standard Model of particle physics. It states that every particle has a (usually more massive) “super” partner.

Although supersymmetry comes in many forms and flavors and took many years to develop, it owes the name “supersymmetry” to a 1974 paper (subscription required). Super comes from “supergauge,” used to describe the high power of gauge operator, and symmetry, because the theory is global rather than local (see paper, subscription required).

The nomenclature for supersymmetric particles was put forward in 1982 in a paper by physicists Ian Hinchliffe and Laurence Littenberg.

To identify the supersymmetric partner particle of a fermion, add the suffix “-ino.” (For example, the supersymmetric partner of a photon would be called a phtoino.) And to identify the partner of a boson, add the prefix “s-.” (For example, the partner of a muon would be a smuon.)

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First results from search for a dark light

Fri, 05/26/2017 - 15:00

The Heavy Photon Search at Jefferson Lab is looking for a hypothetical particle from a hidden “dark sector.”

In 2015, a group of researchers installed a particle detector just half of a millimeter away from an extremely powerful electron beam. The detector could either start them on a new search for a hidden world of particles and forces called the “dark sector”—or its sensitive parts could burn up in the beam.

Earlier this month, scientists presented the results from that very first test run at the Heavy Photon Search collaboration meeting at the US Department of Energy’s Thomas Jefferson National Accelerator Facility. To the scientists’ delight, the experiment is working flawlessly.

Dark sector particles could be the long-sought components of dark matter, the mysterious form of matter thought to be five times more abundant in the universe than regular matter. To be specific, HPS is looking for a dark-sector version of the photon, the elementary “particle of light” that carries the fundamental electromagnetic force in the Standard Model of particle physics.

Analogously, the dark photon would be the carrier of a force between dark-sector particles. But unlike the regular photon, the dark photon would have mass. That’s why it’s also called the heavy photon.

To search for dark photons, the HPS experiment uses a very intense, nearly continuous beam of highly energetic electrons from Jefferson Lab’s CEBAF accelerator. When slammed into a tungsten target, the electrons radiate energy that could potentially produce the mystery particles. Dark photons are believed to quickly decay into pairs of electrons and their antiparticles, positrons, which leave tracks in the HPS detector.

“Dark photons would show up as an anomaly in our data—a very narrow bump on a smooth background from other processes that produce electron-positron pairs,” says Omar Moreno from SLAC National Accelerator Laboratory, who led the analysis of the first data and presented the results at the collaboration meeting.

The challenge is that, due to the large beam energy, the decay products are compressed very narrowly in beam direction. To catch them, the detector must be very close to the electron beam. But not too close—the smallest beam movements could make the beam swerve into the detector. Even if the beam doesn’t directly hit the HPS apparatus, electrons interacting in the target can scatter into the detector and cause unwanted signals. 

The HPS team implemented a number of precautions to make sure their detector could handle the potentially destructive beam conditions. They installed and carefully aligned a system to intercept any large beam motions, made the detector’s support structure movable to bring the detector close to the beam and measure the exact beam position, and installed a feedback system that would shut the beam down if its motions were too large. They also placed their whole setup in vacuum because interactions of the beam with gas molecules would create too much background. Finally, they cooled the detector to negative 30 degrees Fahrenheit to reduce the effects of radiation damage. These measures allowed the team to operate their experiment so close to the beam.

“That’s maybe as close as anyone has ever come to such a particle beam,” says John Jaros, head of the HPS group at SLAC, which built the innermost part of the HPS detector, the Silicon Vertex Tracker. “So, it was fairly exciting when we gradually decreased the distance between the detector and the beam for the first time and saw that everything worked as planned. A large part of that success lies with the beautiful beams Jefferson Lab provided.” 

SLAC’s Mathew Graham, who oversees the HPS analysis group, says, “In addition to figuring out if we can actually do the experiment, the first run also helped us understand the background signals in the experiment and develop the data analysis tools we need for our search for dark photons.”

So far, the team has seen no signs of dark photons. But to be fair, the data they analyzed came from just 1.7 days of accumulated running time. HPS collects data in short spurts when the CLAS experiment, which studies protons and neutrons using the same beam line, is not in use.

A second part of the analysis is still ongoing: The researchers are also closely inspecting the exact location, or vertex, from which an electron-positron pair emerges.

“If a dark photon lives long enough, it might make it out of the tungsten target where it was produced and travel some distance through the detector before it decays into an electron-positron pair,” Moreno says. The detector was specifically designed to observe such a signal.

Jefferson Lab has approved the HPS project for a total of 180 days of experimental time. Slowly but surely, HPS scientists are finding chances to use it.

LHC swings back into action

Tue, 05/23/2017 - 19:21

Protons are colliding once again in the Large Hadron Collider.

This morning at CERN, operators nudged two high-energy beams of protons into a collision course inside the world’s largest and most energetic particle accelerator, the Large Hadron Collider. These first stable beams inside the LHC since the extended winter shutdown usher in another season of particle hunting.

The LHC’s 2017 run is scheduled to last until December 10. The improvements made during the winter break will ensure that scientists can continue to search for new physics and study rare subatomic phenomena. The machine exploits Albert Einstein’s principle that energy and matter are equivalent and enables physicists to transform ordinary protons into the rare massive particles that existed when our universe was still in its infancy.

“Every time the protons collide, it’s like panning for gold,” says Richard Ruiz, a theorist at Durham University. “That’s why we need so much data. It’s very rare that the LHC produces something interesting like a Higgs boson, the subatomic equivalent of a huge gold nugget. We need to find lots of these rare particles so that we can measure their properties and be confident in our results.”

During the LHC’s four-month winter shutdown, engineers replaced one of its main dipole magnets and carried out essential upgrades and maintenance work. Meanwhile, the LHC experiments installed new hardware and revamped their detectors. Over the last several weeks, scientists and engineers have been performing the final checks and preparations for the first “stable beams” collisions.

“There’s no switch for the LHC that instantly turns it on,” says Guy Crockford, an LHC operator. “It’s a long process, and even if it’s all working perfectly, we still need to check and calibrate everything. There’s a lot of power stored in the beam and it can easily damage the machine if we’re not careful.”

In preparation for data-taking, the LHC operations team first did a cold checkout of the circuits and systems without beam and then performed a series of dress rehearsals with only a handful of protons racing around the machine.

“We set up the machine with low intensity beams that are safe enough that we could relax the safety interlocks and make all the necessary tweaks and adjustments,” Crockford says. “We then deliberately made the proton beams unstable to check that all the loose particles were caught cleanly. It’s a long and painstaking process, but we need complete confidence in our settings before ramping up the beam intensity to levels that could easily do damage to the machine.”

The LHC started collisions for physics with only three proton bunches per beam. Over the course of the next month, the operations team will gradually increase the number of proton bunches until they have 2760 per beam. The higher proton intensity greatly increases the rate of collisions, enabling the experiments to collect valuable data at a much faster rate.

“We’re always trying to improve the machine and increase the number of collisions we deliver to the experiments,” Crockford says. “It’s a personal challenge to do a little better every year.”

The facts and nothing but the facts

Tue, 05/16/2017 - 20:00

At a recent workshop on blind analysis, researchers discussed how to keep their expectations out of their results.

Scientific experiments are designed to determine facts about our world. But in complicated analyses, there’s a risk that researchers will unintentionally skew their results to match what they were expecting to find. To reduce or eliminate this potential bias, scientists apply a method known as “blind analysis.”

Blind studies are probably best known from their use in clinical drug trials, in which patients are kept in the dark about—or blind to—whether they’re receiving an actual drug or a placebo. This approach helps researchers judge whether their results stem from the treatment itself or from the patients’ belief that they are receiving it.

Particle physicists and astrophysicists do blind studies, too. The approach is particularly valuable when scientists search for extremely small effects hidden among background noise that point to the existence of something new, not accounted for in the current model. Examples include the much-publicized discoveries of the Higgs boson by experiments at CERN’s Large Hadron Collider and of gravitational waves by the Advanced LIGO detector.

“Scientific analyses are iterative processes, in which we make a series of small adjustments to theoretical models until the models accurately describe the experimental data,” says Elisabeth Krause, a postdoc at the Kavli Institute for Particle Astrophysics and Cosmology, which is jointly operated by Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory. “At each step of an analysis, there is the danger that prior knowledge guides the way we make adjustments. Blind analyses help us make independent and better decisions.”

Krause was the main organizer of a recent workshop at KIPAC that looked into how blind analyses could be incorporated into next-generation astronomical surveys that aim to determine more precisely than ever what the universe is made of and how its components have driven cosmic evolution.

Black boxes and salt

One outcome of the workshop was a finding that there is no one-size-fits-all approach, says KIPAC postdoc Kyle Story, one of the event organizers. “Blind analyses need to be designed individually for each experiment.”

The way the blinding is done needs to leave researchers with enough information to allow a meaningful analysis, and it depends on the type of data coming out of a specific experiment.

A common approach is to base the analysis on only some of the data, excluding the part in which an anomaly is thought to be hiding. The excluded data is said to be in a “black box” or “hidden signal box.”

Take the search for the Higgs boson. Using data collected with the Large Hadron Collider until the end of 2011, researchers saw hints of a bump as a potential sign of a new particle with a mass of about 125 gigaelectronvolts. So when they looked at new data, they deliberately quarantined the mass range around this bump and focused on the remaining data instead.

They used that data to make sure they were working with a sufficiently accurate model. Then they “opened the box” and applied that same model to the untouched region. The bump turned out to be the long-sought Higgs particle.

That worked well for the Higgs researchers. However, as scientists involved with the Large Underground Xenon experiment reported at the workshop, the “black box” method of blind analysis can cause problems if the data you’re expressly not looking at contains rare events crucial to figuring out your model in the first place.

LUX has recently completed one of the world’s most sensitive searches for WIMPs—hypothetical particles of dark matter, an invisible form of matter that is five times more prevalent than regular matter. LUX scientists have done a lot of work to guard LUX against background particles—building the detector in a cleanroom, filling it with thoroughly purified liquid, surrounding it with shielding and installing it under a mile of rock. But a few stray particles make it through nonetheless, and the scientists need to look at all of their data to find and eliminate them.

For that reason, LUX researchers chose a different blinding approach for their analyses. Instead of using a “black box,” they use a process called “salting.”

LUX scientists not involved in the most recent LUX analysis added fake events to the data—simulated signals that just look like real ones. Just like the patients in a blind drug trial, the LUX scientists didn’t know whether they were analyzing real or placebo data. Once they completed their analysis, the scientists that did the “salting” revealed which events were false.

A similar technique was used by LIGO scientists, who eventually made the first detection of extremely tiny ripples in space-time called gravitational waves.

High-stakes astronomical surveys

The Blind Analysis workshop at KIPAC focused on future sky surveys that will make unprecedented measurements of dark energy and the Cosmic Microwave Background—observations that will help cosmologists better understand the evolution of our universe.

Dark energy is thought to be a force that is causing the universe to expand faster and faster as time goes by. The CMB is a faint microwave glow spread out over the entire sky. It is the oldest light in the universe, left over from the time the cosmos was only 380,000 years old.

To shed light on the mysterious properties of dark energy, the Dark Energy Science Collaboration is preparing to use data from the Large Synoptic Survey Telescope, which is under construction in Chile. With its unique 3.2-gigapixel camera, LSST will image billions of galaxies, the distribution of which is thought to be strongly influenced by dark energy.

“Blinding will help us look at the properties of galaxies picked for this analysis independent of the well-known cosmological implications of preceding studies,” DESC member Krause says. One way the collaboration plans on blinding its members to this prior knowledge is to distort the images of galaxies before they enter the analysis pipeline.

Not everyone in the scientific community is convinced that blinding is necessary. Blind analyses are more complicated to design than non-blind analyses and take more time to complete. Some scientists participating in blind analyses inevitably spend time looking at fake data, which can feel like a waste.

Yet others strongly advocate for going blind. KIPAC researcher Aaron Roodman, a particle-physicist-turned-astrophysicist, has been using blinding methods for the past 20 years.

“Blind analyses have already become pretty standard in the particle physics world,” he says. “They’ll be also crucial for taking bias out of next-generation cosmological surveys, particularly when the stakes are high. We’ll only build one LSST, for example, to provide us with unprecedented views of the sky.”

CERN unveils new linear accelerator

Tue, 05/09/2017 - 16:31

Linac 4 will replace an older accelerator as the first step in the complex that includes the LHC.

At a ceremony today, CERN European research center inaugurated its newest accelerator.

Linac 4 will eventually become the first step in CERN’s accelerator chain, delivering proton beams to a wide range of experiments, including those at the Large Hadron Collider.

After an extensive testing period, Linac 4 will be connected to CERN’s accelerator complex during a long technical shutdown in 2019-20. Linac 4 will replace Linac 2, which was put into service in 1978. Linac 4 will feed the CERN accelerator complex with particle beams of higher energy.

“We are delighted to celebrate this remarkable accomplishment,” says CERN Director General Fabiola Gianotti. “Linac 4 is a modern injector and the first key element of our ambitious upgrade program, leading to the High-Luminosity LHC. This high-luminosity phase will considerably increase the potential of the LHC experiments for discovering new physics and measuring the properties of the Higgs particle in more detail.”

“This is an achievement not only for CERN, but also for the partners from many countries who contributed in designing and building this new machine,” says CERN Director for Accelerators and Technology Frédérick Bordry. “We also today celebrate and thank the wide international collaboration that led this project, demonstrating once again what can be accomplished by bringing together the efforts of many nations.”

The linear accelerator is the first essential element of an accelerator chain. In the linear accelerator, the particles are produced and receive the initial acceleration. The density and intensity of the particle beams are also shaped in the linac. Linac 4 is an almost 90-meter-long machine sitting 12 meters below the ground. It took nearly 10 years to build it.

Linac 4 will send negative hydrogen ions, consisting of a hydrogen atom with two electrons, to CERN’s Proton Synchrotron Booster, which further accelerates the negative ions and removes the electrons. Linac 4 will bring the beam up to an energy of 160 million electronvolts, more than 3 times the energy of its predecessor. The increase in energy, together with the use of hydrogen ions, will enable doubling the beam intensity delivered to the LHC, contributing to an increase in the luminosity of the LHC by 2021.

Luminosity is a parameter indicating the number of particles colliding within a defined amount of time. The peak luminosity of the LHC is planned to be increased by a factor of 5 by the year 2025. This will make it possible for the experiments to accumulate about 10 times more data over the period 2025 to 2035 than before.

Editor's note: This article is based on a CERN press release.

Understanding the unknown universe

Tue, 05/09/2017 - 16:26

The authors of We Have No Idea remind us that there are still many unsolved mysteries in science.

What is dark energy? Why aren’t we made of antimatter? How many dimensions are there? 

These are a few of the many unanswered questions that Jorge Cham, creator of the online comic Piled Higher and Deeper, and Daniel Whiteson, an experimental particle physicist at the University of California, Irvine, explain in their new book, We Have No Idea. In the process, they remind readers of one key point: When it comes to our universe, there’s a lot we still don’t know. 

Jorge Cham

The duo started working together in 2008 after Whiteson reached out to Cham, asking if he’d be willing to help create physics cartoons. “I always thought physics was well connected to the way comics work,” Whiteson says. “Because, what’s a Feynman diagram but a little cartoon of particles hitting each other?” (Feynman diagrams are pictures commonly used in particle physics papers that represent the interactions of subatomic particles.)

Daniel Whiteson

Before working on this book, the pair made a handful of popular YouTube videos on topics like dark matter, extra dimensions and the Higgs boson. Many of these subjects are also covered in We Have No Idea.

One of the main motivators of this latest project was to address a “certain apathy toward science,” Cham says. “I think we both came into it having this feeling that the general public either thinks scientists have everything figured out, or they don't really understand what scientists are doing.” 

To get at this issue, the pair focused on topics that even someone without a science background could find compelling. “You don’t need 10 years of physics background to know [that] questions about how the universe started or what it’s made of are interesting,” Whiteson says. “We tried to find questions that were gut-level approachable.”

Another key theme of the book, the authors say, is the line between what science can and cannot tell us. While some of the possible solutions to the universe’s mysteries have testable predictions, others (such as string theory) currently do not. “We wanted questions that were accessible yet answerable,” says Whiteson. “We wanted to show people that there were deep, basic, simple questions that we all had, but that the answers were out there.” 

Many scientists are hard at work trying to fill the gaping holes in our knowledge about the universe. Particle physicists, for example, are exploring a number of these questions, such as those about the nature of antimatter and mass.

Artwork by Jorge Cham

Some lines of inquiry have brought different research communities together. Dark matter searches, for example, were primarily the realm of cosmologists, who probe large-scale structures of the universe. However, as the focus shifted to finding out what particle—or particles—dark matter was made of, this area of study started to attract astrophysicists as well. 

Why are people trying to answer these questions? “I think science is an expression of humanity and our curiosity to know the answers to basic questions we ask ourselves: Who are we? Why are we here? How does the world work?” Whiteson says. “On the other hand, questions like these lead to understanding, and understanding leads to being able to have greater power over the environment to solve our problems.

In the very last chapter of the book, the authors explain the idea of a “testable universe,” or the parts of the universe that fall within the bounds of science. In the Stone Ages, when humans had very few tools at their disposal, the testable universe was very small. But it increased as people built telescopes, satellites and particle colliders, and it continues to expand with ongoing advances in science and technology. “That’s the exciting thing,” Cham says. “Our ability to answer these questions is growing.” 

Some mysteries of the universe still live in the realm of philosophy. But tomorrow, next year or a thousand years from now, a scientist may come along and devise an experiment that will be able to find the answers.   

“We’re in a special place in history when most of the world seems explained,” Whiteson says. Thousands of years ago, basic questions, such as why fire burns or where rain comes from, were still largely a mystery. “These days, all those mysteries seem answered, but the truth is, there’s a lot of mysteries left. [If] you want to make a massive imprint on human intellectual history, there’s plenty of room for that.”

Sterile neutrino search hits roadblock at reactors

Thu, 05/04/2017 - 15:00

A new result from the Daya Bay collaboration reveals both limitations and strengths of experiments studying antineutrinos at nuclear reactors.

As nuclear reactors burn through fuel, they produce a steady flow of particles called neutrinos. Neutrinos interact so rarely with other matter that they can flow past the steel and concrete of a power plant’s containment structures and keep on moving through anything else that gets in their way.

Physicists interested in studying these wandering particles have taken advantage of this fact by installing neutrino detectors nearby. A recent result using some of these detectors demonstrated both their limitations and strengths.

The reactor antineutrino anomaly

In 2011, a group of theorists noticed that several reactor-based neutrino experiments had been publishing the same, surprising result: They weren’t detecting as many neutrinos as they thought they would.

Or rather, to be technically correct, they weren’t seeing as many antineutrinos as they thought they would; nuclear reactors actually produce the antimatter partners of the elusive particles. About 6 percent of the expected antineutrinos just weren’t showing up. They called it “the reactor antineutrino anomaly.”

The case of the missing neutrinos was a familiar one. In the 1960s, the Davis experiment located in Homestake Mine in South Dakota reported a shortage of neutrinos coming from processes in the sun. Other experiments confirmed the finding. In 2001, the Sudbury Neutrino Observatory in Ontario demonstrated that the missing neutrinos weren’t missing at all; they had only undergone a bit of a costume change.

Neutrinos come in three types. Scientists discovered that neutrinos could transform from one type to another. The missing neutrinos had changed into a different type of neutrino that the Davis experiment couldn’t detect.

Since 2011, scientists have wondered whether the reactor antineutrino anomaly was a sign of an undiscovered type of neutrino, one that was even harder to detect, called a sterile neutrino.

A new result from the Daya Bay experiment in China not only casts doubt on that theory, it also casts doubt on the idea that scientists understand their model of reactor processes well enough at this time to use it to search for sterile neutrinos.

The word from Daya Bay

The Daya Bay experiment studies antineutrinos coming from six nuclear reactors on the southern coast of China, about 35 miles northeast of Hong Kong. The reactors are powered by the fission of uranium. Over time, the amount of uranium inside the reactor decreases while the amount of plutonium increases. The fuel is changed—or cycled—about every 18 months.

The main goal of the Daya Bay experiment was to look for the rarest of the known neutrino oscillations. It did that, making a groundbreaking discovery after just nine weeks of data-taking.

But that wasn’t the only goal of the experiment. “We realized right from the beginning that it is important for Daya Bay to address as many interesting physics problems as possible,” says Daya Bay co-spokesperson Kam-Biu Luk of the University of California, Berkeley and the US Department of Energy’s Lawrence Berkeley National Laboratory.

For this result, Daya Bay scientists took advantage of their enormous collection of antineutrino data to expand their investigation to the reactor antineutrino anomaly.

Using data from more than 2 million antineutrino interactions and information about when the power plants refreshed the uranium in each reactor, Daya Bay physicists compared the measurements of antineutrinos coming from different parts of the fuel cycle: early ones dominated by uranium through later ones dominated by both uranium and plutonium.

In theory, the type of fuel producing the antineutrinos should not affect the rate at which they transform into sterile neutrinos. According to Bob Svoboda, chair of the Department of Physics at the University of California, Davis, “a neutrino wouldn’t care how it got made.” But Daya Bay scientists found that the shortage of antineutrinos existed only in processes dominated by uranium.

Their conclusion is that, once again, the missing neutrinos aren’t actually missing. This time, the problem of the missing antineutrinos seems to stem from our understanding of how uranium burns in nuclear power plants. The predictions for how many antineutrinos the scientists should detect may have been overestimated.

“Most of the problem appears to come from the uranium-235 model (uranium-235 is a fissile isotope of uranium), not from the neutrinos themselves,” Svoboda says. “We don’t fully understand uranium, so we have to take any anomaly we measured with a grain of salt.”

This knock against the reactor antineutrino anomaly does not disprove the existence of sterile neutrinos. Other, non-reactor experiments have seen different possible signs of their influence. But it does put a damper on the only evidence of sterile neutrinos to have come from reactor experiments so far.

Other reactor neutrino experiments, such as NEOS in South Korea and PROSPECT in the United States will fill in some missing details. NEOS scientists directly measured antineutrinos coming from reactors in the Hanbit nuclear power complex using a detector placed about 80 feet away, a distance some scientists believe is optimal for detecting sterile neutrinos should they exist. PROSPECT scientists will make the first precision measurement of antineutrinos coming from a highly enriched uranium core, one that does not produce plutonium as it burns.

A silver lining

The Daya Bay result offers the most detailed demonstration yet of scientists’ ability to use neutrino detectors to peer inside running nuclear reactors.

“As a study of reactors, this is a tour de force,” says theorist Alexander Friedland of SLAC National Accelerator Laboratory. “This is an explicit demonstration that the composition of the reactor fuel has an impact on the neutrinos.”

Some scientists are interested in monitoring nuclear power plants to find out if nuclear fuel is being diverted to build nuclear weapons.

“Suppose I declare my reactor produces 100 kilograms of plutonium per year,” says Adam Bernstein of the University of Hawaii and Lawrence Livermore National Laboratory. “Then I operate it in a slightly different way, and at the end of the year I have 120 kilograms.” That 20-kilogram surplus, left unmeasured, could potentially be moved into a weapons program.

Current monitoring techniques involve checking what goes into a nuclear power plant before the fuel cycle begins and then checking what comes out after it ends. In the meantime, what happens inside is a mystery.

Neutrino detectors allow scientists to understand what’s going on in a nuclear reactor in real time.

Scientists have known for decades that neutrino detectors could be useful for nuclear nonproliferation purposes. Scientists studying neutrinos at the Rovno Nuclear Power Plant in Ukraine first demonstrated that neutrino detectors could differentiate between uranium and plutonium fuel.

Most of the experiments have done this by looking at changes in the aggregate number of antineutrinos coming from a detector. Daya Bay showed that neutrino detectors could track the plutonium inventory in nuclear fuel by studying the energy spectrum of antineutrinos produced.

“The most likely use of neutrino detectors in the near future is in so-called ‘cooperative agreements,’ where a $20-million-scale neutrino detector is installed in the vicinity of a reactor site as part of a treaty,” Svoboda says. “The site can be monitored very reliably without having to make intrusive inspections that bring up issues of national sovereignty.”

Luk says he is dubious that the idea will take off, but he agrees that Daya Bay has shown that neutrino detectors can give an incredibly precise report. “This result is the best demonstration so far of using a neutrino detector to probe the heartbeat of a nuclear reactor.”

Mystery glow of Milky Way likely not dark matter

Tue, 05/02/2017 - 18:40

According to the Fermi LAT collaboration, the galaxy’s excessive gamma-ray glow likely comes from pulsars, the remains of collapsed ancient stars.

A mysterious gamma-ray glow at the center of the Milky Way is most likely caused by pulsars, the incredibly dense, rapidly spinning cores of collapsed ancient stars that were up to 30 times more massive than the sun.

That’s the conclusion of a new analysis by an international team of astrophysicists on the Fermi LAT collaboration. The findings cast doubt on previous interpretations of the signal as a potential sign of dark matter, a form of matter that accounts for 85 percent of all matter in the universe but that so far has evaded detection.

“Our study shows that we don’t need dark matter to understand the gamma-ray emissions of our galaxy,” says Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and the US Department of Energy's SLAC National Accelerator Laboratory. “Instead, we have identified a population of pulsars in the region around the galactic center, which sheds new light on the formation history of the Milky Way.”

Di Mauro led the analysis, which looked at the glow with the Large Area Telescope on NASA’s Fermi Gamma-ray Space Telescope, which has been orbiting Earth since 2008. The LAT, a sensitive “eye” for gamma rays, the most energetic form of light, was conceived of and assembled at SLAC, which also hosts its operations center.

The collaboration’s findings, submitted to The Astrophysical Journal for publication, are available as a preprint.   

A mysterious glow

Dark matter is one of the biggest mysteries of modern physics. Researchers know that dark matter exists because it bends light from distant galaxies and affects how galaxies rotate. But they don’t know what the substance is made of. Most scientists believe it’s composed of yet-to-be-discovered particles that almost never interact with regular matter other than through gravity, making it very hard to detect them.

One way scientific instruments might catch a glimpse of dark matter particles is when the particles either decay or collide and destroy each other. “Widely studied theories predict that these processes would produce gamma rays,” says Seth Digel, head of KIPAC’s Fermi group. “We search for this radiation with the LAT in regions of the universe that are rich in dark matter, such as the center of our galaxy.”

Previous studies have indeed shown that there are more gamma rays coming from the galactic center than expected, fueling some scientific papers and media reports that suggest the signal might hint at long-sought dark matter particles. However, gamma rays are produced in a number of other cosmic processes, which must be ruled out before any conclusion about dark matter can be drawn. This is particularly challenging because the galactic center is extremely complex, and astrophysicists don’t know all the details of what’s going on in that region. 

Most of the Milky Way’s gamma rays originate in gas between the stars that is lit up by cosmic rays, charged particles produced in powerful star explosions called supernovae. This creates a diffuse gamma-ray glow that extends throughout the galaxy. Gamma rays are also produced by supernova remnants, pulsars—collapsed stars that emit “beams” of gamma rays like cosmic lighthouses—and more exotic objects that appear as points of light.  

“Two recent studies by teams in the US and the Netherlands have shown that the gamma-ray excess at the galactic center is speckled, not smooth as we would expect for a dark matter signal,” says KIPAC’s Eric Charles, who contributed to the new analysis. “Those results suggest the speckles may be due to point sources that we can’t see as individual sources with the LAT because the density of gamma-ray sources is very high and the diffuse glow is brightest at the galactic center.”

Remains of ancient stars

The new study takes the earlier analyses to the next level, demonstrating that the speckled gamma-ray signal is consistent with pulsars.

“Considering that about 70 percent of all point sources in the Milky Way are pulsars, they were the most likely candidates,” Di Mauro says. “But we used one of their physical properties to come to our conclusion. Pulsars have very distinct spectra—that is, their emissions vary in a specific way with the energy of the gamma rays they emit. Using the shape of these spectra, we were able to model the glow of the galactic center correctly with a population of about 1,000 pulsars and without introducing processes that involve dark matter particles.”

The team is now planning follow-up studies with radio telescopes to determine whether the identified sources are emitting their light as a series of brief light pulses—the trademark that gives pulsars their name.

Discoveries in the halo of stars around the center of the galaxy, the oldest part of the Milky Way, also reveal details about the evolution of our galactic home, just as ancient remains teach archaeologists about human history.

“Isolated pulsars have a typical lifetime of 10 million years, which is much shorter than the age of the oldest stars near the galactic center,” Charles says. “The fact that we can still see gamma rays from the identified pulsar population today suggests that the pulsars are in binary systems with companion stars, from which they leach energy. This extends the life of the pulsars tremendously.”    

Dark matter remains elusive

The new results add to other data that are challenging the interpretation of the gamma-ray excess as a dark matter signal.

“If the signal were due to dark matter, we would expect to see it also at the centers of other galaxies,” Digel says. “The signal should be particularly clear in dwarf galaxies orbiting the Milky Way. These galaxies have very few stars, typically don’t have pulsars and are held together because they have a lot of dark matter. However, we don’t see any significant gamma-ray emissions from them.”

The researchers believe that a recently discovered strong gamma-ray glow at the center of the Andromeda galaxy, the major galaxy closest to the Milky Way, may also be caused by pulsars rather than dark matter. 

But the last word may not have been spoken. Although the Fermi-LAT team studied a large area of 40 degrees by 40 degrees around the Milky Way’s galactic center (the diameter of the full moon is about half a degree), the extremely high density of sources in the innermost four degrees makes it very difficult to see individual ones and rule out a smooth, dark matter-like gamma-ray distribution, leaving limited room for dark matter signals to hide.  

This work was funded by NASA and the DOE Office of Science, as well as agencies and institutes in France, Italy, Japan and Sweden.

Editor's note: A version of this article was originally published by SLAC National Accelerator Laboratory.

#AskSymmetry Twitter chat with Tulika Bose

Fri, 04/28/2017 - 21:59

See Boston University physicist Tulika Bose's answers to readers’ questions about research at the Large Hadron Collider.

[View the story "#AskSymmetry Twitter chat with Tulika Bose 4/28/17" on Storify]