CERN News

Subscribe to CERN News feed
Updated: 29 sec ago

Crab cavities: colliding protons head-on

Fri, 12/08/2017 - 09:59

One of the first two crab cavities during construction in a clean room at CERN. (Image: Ulysse Fichet/CERN)

They won’t pinch you and you won’t find them on the beach. The name of the new radio-frequency crab cavities has nothing to do with their appearance and is merely illustrative of the effect they will have on circulating proton bunches.

Crab cavities will help increase the luminosity of collisions in the High-Luminosity LHC (HL-LHC) – the future upgrade of the LHC planned for after 2025. The luminosity of a collider is proportional to the number of collisions that occur in a given amount of time. The higher the luminosity, the more collisions, and the more data the experiments can gather to allow them to observe rare processes.

At present, two superconducting crab cavities have been manufactured at CERN and inserted into a specially designed cryostat, which will keep them at their operating temperature of two kelvin. Currently in their final stages of testing, they will be installed in the Super Proton Synchrotron (SPS) during this year’s winter technical stop. In 2018, they will be tested with a proton beam for the first time. 

The assembly of the crab cavity housing, a cryostat that will serve as a high-performance thermos flask, reducing the heat load and keeping the cavities at their operating temperature. (Image: Maximilien Brice/CERN)

The beams in the LHC are made of bunches, each containing billions of protons. They are similar to trains with carriages full of billions of passengers. In the LHC, the two counter-circulating proton beams meet at a small crossing angle at the collision point of the experiments.

What makes the crab cavities special is their ability to “tilt” the proton bunches in each beam, maximising their overlap at the collision point. Тhis way every single proton in the bunch is forced to pass through the whole length of the opposite bunch, which increases the probability that it will collide with another particle. After being tilted, the motion of the proton bunches appears to be sideways – just like a crab.

An illustration of the effect of the crab cavities on the proton bunches. (Image: CERN)

Find out more about the crab cavities in the video below.

Rama Calaga, the radio-frequency physicist behind the technology, Ofelia Capatina, deputy leader of the crab cavities project, and Lucio Rossi, leader of the High-Luminosity LHC project, explain the principle of the crab cavities. (Video: Polar Media and CERN Audiovisual Productions)

At the LHC, tomorrow is already here

Tue, 12/05/2017 - 11:24

The CERN Control Centre in 2017, from where all the Laboratory's accelerators and technical infrastructure are controlled. The accelerator complex and the LHC produced a record amount of data in 2017. (Image: Julien Ordan/CERN)

On Monday, 4 December at 4.00 a.m., the accelerator operators hit the stop button on the accelerator complex and the Large Hadron Collider for their usual winter break. But while the machines are hibernating, there’s no rest for the humans, as CERN teams will be busy with all the maintenance and upgrade work required before the machines are restarted in the spring. 

The LHC has ended the year with yet another luminosity record, having produced 50 inverse femtobarns of data, i.e. 5 million billion collisions, in 2017. But the accelerator hasn’t just produced lots of data for the physics programmes. 

Before the technical stop, a number of new techniques for increasing the luminosity of the machine were tested. These techniques are mostly being developed for the LHC’s successor, the High-Luminosity LHC. With a planned start-up date of 2026, the High-Luminosity LHC will produce five to ten times as many collisions as the current LHC. To do this, it will be kitted out with new equipment and will use a new optics scheme, based on ATS (Achromatic Telescopic Squeezing), a configuration that was tested this year at the LHC. 

Handling beams of particles is a bit like handling beams of light. In an accelerator, dipole magnets act like mirrors, guiding the beams around the bends. Quadrupole magnets act alternately like concave or convex lenses, keeping the beams in line transversally, but mainly focusing them as much as possible at the interaction points of the experiments. Corrector magnets (hexapoles) correct chromatic aberrations (a bit like corrective lenses for astigmatism). Configuring the optics of an accelerator is all about combining the strengths of these different magnets.

One particularly efficient approach to increasing luminosity, and therefore the number of collisions, is to reduce the size of the beam at the interaction points, or in other words to compress the bunches of particles as much as possible. In the High-Luminosity LHC, more powerful quadrupole magnets with larger apertures, installed either side of the experiments, will focus the bunches before collision. However, for these magnets to be as effective as possible, the beam must first be considerably expanded: a bit like a stretching a spring as much as possible so that it retracts as much as possible. And this is where the new configuration comes in. Instead of just using the quadrupole magnets either side of the collision points, the ATS system also makes use of magnets situated further away from the experiments in the machine, transforming seven kilometres of the accelerator into a giant focusing system. 

Graph showing the integrated luminosity over the various runs of the LHC. In 2017, the LHC produced 50 inverse femtobarns of data, the equivalent of 5 million billion collisions. (Image: CERN)

These techniques have been used in part this year at the LHC and will be used even more during future runs. “The heart of the High-Luminosity LHC is already beating in the LHC,” explains Stéphane Fartoukh, the physicist who came up with the new concept.  “The latest tests, carried out last week, have once again proved the reliability of the scheme and demonstrated other potential applications, sometimes beyond our initial expectations.

For further information:

How to produce the purest argon ever?

Thu, 11/30/2017 - 16:16

ARIA’s modules are being leak-tested at CERN before travelling to Sardinia, Italy. The top, bottom and one standard column module have now been lined up horizontally to test their alignment. (Image: J. Ordan/CERN)

Producing the purest argon ever made is no mean feat, in fact it needs a column 26 metres taller than the Eiffel Tower.

CERN is part of a project, called ARIA, to construct a 350-metre-tall distillation tower that will be used to purify liquid argon for scientific and, in a second phase, medical use.

The full tower, composed of 28 identical modules plus a top (condenser) and a bottom (re-boiler) special module, will be installed in a disused mine site in Sardinia, Italy.

The project is was initiated to supply the purest argon possible to the international dark matter experiment DarkSide at INFN’s Gran Sasso National Laboratories. DarkSide is a dual-phase liquid-argon time-projection chamber that aims to detect the possible passage of a dark matter particle in the form of a Weakly Interacting Massive Particle (WIMP) when it hits the argon nuclei contained in the detector. Since this WIMP-nuclei interaction is predicted to be extremely rare, the detector must contain only the purest argon possible, so as not to accidentally produce a spurious signal.

ARIA has been designed to produce this extra-pure argon. Atmospheric argon contains many “impurities” such as water, oxygen, krypton and argon-39, an isotope of argon, which are all sources of unwanted signals. Argon from underground sources is already depleted from the argon-39 isotope by a factor of 1400, but this is still not enough for dark-matter research. ARIA is designed to purify underground argon by a further factor of 100.

For more information, read this article.

A very special run for the LHCb experiment

Thu, 11/30/2017 - 09:19

The LHCb detector in open configuration. (Image: Anna Pantelia/CERN)

For the first time, the LHCb experiment at CERN has collected data simultaneously in collider and in fixed-target modes. With this, the LHCb special run is even more special.

The past two weeks have been devoted to special runs of the Large Hadron Collider (LHC), at the end of the LHC 2017 proton run and before the winter shutdown. One run involved proton collisions at an energy of 5.02 TeV, mainly to set a reference to compare with lead-ion collision data. What was exceptional this year is that a tiny quantity of neon gas was injected into the beam pipe near the LHCb experiment’s interaction point. This allowed physicists to collect proton-neon at the same time as proton-proton collision data.

When (noble) gases are injected into the beam pipe to collide with protons, the LHCb experiment is in “fixed-target” mode, in contrast to the standard “collider” mode. But unlike traditional fixed target experiments, where the beam of accelerated particles is directed at a dense solid or liquid target, here LHC protons are colliding with a handful of neon nuclei injected near the collision point and floating in the beam pipe. These nuclei slightly pollute the almost perfect LHC vacuum, but the conditions they create – where pressure is in the order of 10-7 millibar – are still considered to be typical of ultra-high vacuum environments.

There are two main reasons to collect proton-gas collision data at the LHC. On one hand, these data help understand nuclear effects (i.e. depending on the type of nuclei involved in the collisions), affecting the production of specific types of particles (J/ψ and D0 mesons), whose suppressed production is considered to be the hallmark of the quark-gluon plasma. The quark-gluon plasma is the state in which the matter filling the universe a few millionths of a second after the Big Bang was , when protons and neutrons had not yet formed, composed of quarks not binding together and then free to move on their own.  

On the other hand, proton-neon interactions are important to also study cosmic rays – highly energetic particles, mostly protons, coming from outside the Solar System – when they collide with nuclei in the Earth’s atmosphere. Neon is one of the components of the Earth’s atmosphere and it is very similar in terms of nuclear size to the much more abundant nitrogen and oxygen.

This gas-injection technique was originally designed to measure the brightness of the accelerator's beams, but its potential was quickly recognised by the LHCb physicists and it is now also being used for dedicated physics measurements. In 2015 and 2016, the LHCb experiment already performed special proton-helium, proton-neon and proton-argon runs. In October this year, for eight hours only, the LHC accelerated and collided xenon nuclei, allowing the four large LHC experiments to record xenon-xenon collisions for the first time.

This recent 11-day proton-neon run will allow physicists to collect a dataset that is 100 times larger than all proton-neon collision data collected until now at the LHC, and the first results of the analyses are foreseen for next year.

Find out more on the LHCb website

AWAKE: Closer to a breakthrough acceleration technology

Fri, 11/24/2017 - 15:48

The electron source and electron beam line of AWAKE have just been installed. (Image: Maximilien Brice, Julien Ordan/CERN)

We are one step closer to testing a breakthrough technology for particle acceleration. The final three key parts of AWAKE have just been put in place: its electron source, electron beam line and electron spectrometer. This marks the end of the installation phase of the Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE), a proof-of-principle experiment at CERN that is developing a new technique for accelerating particles.

The accelerators currently in use rely on electric fields generated by radiofrequency (RF) cavities to accelerate charged particles by giving them a “kick”. In AWAKE, a beam of electrons will “surf” waves of electric charges, or wakefields. These waves are created when a beam of protons is injected into the heart of AWAKE, a 10-metre plasma cell full of ionised gas. When the protons travel through the plasma, they attract free electrons, which generate wakefields. A second particle beam, this time of electrons, is injected into the right phase behind the proton beam. As a result, it feels the wakefield and is accelerated, just like a surfer riding a wave.

After exiting the plasma cell, the electrons will pass through a dipole magnet, which will curve their path. More energetic particles will get a smaller curvature. As well as the electron beam line, another new component is the scintillator that awaits the electrons at the end of the dipole, showing whether or not they have been accelerated. Essentially, this is a screen that lights up whenever a charged particle passes through it. Successfully accelerated electrons will be bent to a lesser degree by the magnetic field and will appear on one side of the scintillator.

Ans Pardons, integration and installation coordinator of AWAKE, beside the one-metre-wide scintillator. (Image: Maximilien Brice, Julien Ordan/CERN)

From now until the end of 2017, the whole AWAKE experiment, including the electron source and the electron beam line, will be being commissioned and prepared for a very important year ahead. “It is very important to first create a high-quality electron beam with the correct energy and intensity, and then to successfully send it through the electron beam line and the plasma cell,” explains Ans Pardons, integration and installation coordinator of AWAKE.

The first milestone was reached in December 2016, when the first data showed that the wakefields had been successfully generated. After a very successful 2017 run, it is now time for AWAKE’s next big step. Next year will be fully dedicated to proving that the acceleration of electrons in the wake of proton bunches is possible.

AWAKE's new electron line (Image: Maximilien Brice, Julien Ordan)

AWAKE: Closer to a breakthrough acceleration technology

Fri, 11/24/2017 - 15:48

The electron source and electron beam line of AWAKE have just been installed. (Image: Maximilien Brice, Julien Ordan/CERN)

We are one step closer to testing a breakthrough technology for particle acceleration. The final three key parts of AWAKE have just been put in place: its electron source, electron beam line and electron spectrometer. This marks the end of the installation phase of the Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE), a proof-of-principle experiment at CERN that is developing a new technique for accelerating particles.

The accelerators currently in use rely on electric fields generated by radiofrequency (RF) cavities to accelerate charged particles by giving them a “kick”. In AWAKE, a beam of electrons will “surf” waves of electric charges, or wakefields. These waves are created when a beam of protons is injected into the heart of AWAKE, a 10-metre plasma cell full of ionised gas. When the protons travel through the plasma, they attract free electrons, which generate wakefields. A second particle beam, this time of electrons, is injected into the right phase behind the proton beam. As a result, it feels the wakefield and is accelerated, just like a surfer riding a wave.

After exiting the plasma cell, the electrons will pass through a dipole magnet, which will curve their path. More energetic particles will get a smaller curvature. As well as the electron beam line, another new component is the scintillator that awaits the electrons at the end of the dipole, showing whether or not they have been accelerated. Essentially, this is a screen that lights up whenever a charged particle passes through it. Successfully accelerated electrons will be bent to a lesser degree by the magnetic field and will appear on one side of the scintillator.

Ans Pardons, integration and installation coordinator of AWAKE, beside the one-metre-wide scintillator. (Image: Maximilien Brice, Julien Ordan/CERN)

From now until the end of 2017, the whole AWAKE experiment, including the electron source and the electron beam line, will be being commissioned and prepared for a very important year ahead. “It is very important to first create a high-quality electron beam with the correct energy and intensity, and then to successfully send it through the electron beam line and the plasma cell,” explains Ans Pardons, integration and installation coordinator of AWAKE.

The first milestone was reached in December 2016, when the first data showed that the wakefields had been successfully generated. After a very successful 2017 run, it is now time for AWAKE’s next big step. Next year will be fully dedicated to proving that the acceleration of electrons in the wake of proton bunches is possible.

AWAKE's new electron line (Image: Maximilien Brice, Julien Ordan)

First light for pioneering SESAME light source

Thu, 11/23/2017 - 11:07

SESAME XAFS/XRF beamline scientist, Messaoud Harfouche, points out SESAME’s first monochromatic light. (Image: SESAME)

At 10:50 yesterday morning scientists at the pioneering SESAME light source saw First Monochromatic Light through the XAFS/XRF (X-ray absorption fine structure/X-ray fluorescence) spectroscopy beamline, signalling the start of the laboratory’s experimental programme. This beamline, SESAME’s first to come on stream, delivers X-ray light that will be used to carry out research in areas ranging from solid state physics to environmental science and archaeology.

“After years of preparation, it’s great to see light on target,” said XAFS/XRF beamline scientist Messaoud Harfouche. “We have a fantastic experimental programme ahead of us, starting with an experiment to investigate heavy metals contaminating soils in the region.”

The initial research programme will be carried out at two beamlines, the XAFS/XRF beamline and the Infrared (IR) spectromicroscopy beamline that is scheduled to join the XAFS/XRF beamline this year. Both have specific characteristics that make them appropriate for various areas of research. A third beamline, devoted to materials science, will come on stream in 2018.


Inside SESAME's ring (Image: Noemi Caraban/CERN)

“Our first three beamlines already give SESAME a wide range of research options to fulfil the needs of our research community,” said SESAME Scientific Director Giorgio Paolucci, “the future for light source research in the Middle East and neighbouring countries is looking very bright!”

First Light is an important step in the commissioning process of a new synchrotron light source, but it is nevertheless just one step on the way to full operation. The SESAME synchrotron is currently operating with a beam current of just over 80 milliamps, while the design value is 400 milliamps. Over the coming weeks and months as experiments get underway, the current will be gradually increased.

“SESAME is a major scientific and technological addition to research and education in the Middle East and beyond,” said Director of SESAME, Khaled Toukan. “Jordan supported the project financially and politically since its inception in 2004 for the benefit of science and peace in the region. The young scientists, physicists, engineers and administrators who have built SESAME, come for the first time from this part of the world.”

Among the subjects likely to be studied in early experiments are environmental pollution with a view to improving public health, as well as studies aimed at identifying new drugs for cancer therapy, and cultural heritage studies ranging from bioarcheology – the study of our ancestors – to investigations of ancient manuscripts.

“On behalf of the SESAME Council, I’d like to congratulate the SESAME staff on this wonderful milestone,” said President of the Council, Rolf Heuer. “SESAME is a great addition to the region’s research infrastructure, allowing scientists from the region access to the kind of facility that they previously had to travel to Europe or the US to use.”

 

 

 

First light for pioneering SESAME light source

Thu, 11/23/2017 - 11:07

SESAME XAFS/XRF beamline scientist, Messaoud Harfouche, points out SESAME’s first monochromatic light. (Image: SESAME)

At 10:50 yesterday morning scientists at the pioneering SESAME light source saw First Monochromatic Light through the XAFS/XRF (X-ray absorption fine structure/X-ray fluorescence) spectroscopy beamline, signalling the start of the laboratory’s experimental programme. This beamline, SESAME’s first to come on stream, delivers X-ray light that will be used to carry out research in areas ranging from solid state physics to environmental science and archaeology.

“After years of preparation, it’s great to see light on target,” said XAFS/XRF beamline scientist Messaoud Harfouche. “We have a fantastic experimental programme ahead of us, starting with an experiment to investigate heavy metals contaminating soils in the region.”

The initial research programme will be carried out at two beamlines, the XAFS/XRF beamline and the Infrared (IR) spectromicroscopy beamline that is scheduled to join the XAFS/XRF beamline this year. Both have specific characteristics that make them appropriate for various areas of research. A third beamline, devoted to materials science, will come on stream in 2018.


Inside SESAME's ring (Image: Noemi Caraban/CERN)

“Our first three beamlines already give SESAME a wide range of research options to fulfil the needs of our research community,” said SESAME Scientific Director Giorgio Paolucci, “the future for light source research in the Middle East and neighbouring countries is looking very bright!”

First Light is an important step in the commissioning process of a new synchrotron light source, but it is nevertheless just one step on the way to full operation. The SESAME synchrotron is currently operating with a beam current of just over 80 milliamps, while the design value is 400 milliamps. Over the coming weeks and months as experiments get underway, the current will be gradually increased.

“SESAME is a major scientific and technological addition to research and education in the Middle East and beyond,” said Director of SESAME, Khaled Toukan. “Jordan supported the project financially and politically since its inception in 2004 for the benefit of science and peace in the region. The young scientists, physicists, engineers and administrators who have built SESAME, come for the first time from this part of the world.”

Among the subjects likely to be studied in early experiments are environmental pollution with a view to improving public health, as well as studies aimed at identifying new drugs for cancer therapy, and cultural heritage studies ranging from bioarcheology – the study of our ancestors – to investigations of ancient manuscripts.

“On behalf of the SESAME Council, I’d like to congratulate the SESAME staff on this wonderful milestone,” said President of the Council, Rolf Heuer. “SESAME is a great addition to the region’s research infrastructure, allowing scientists from the region access to the kind of facility that they previously had to travel to Europe or the US to use.”

 

 

 

CERN surprises Swiss expo visitors

Tue, 11/21/2017 - 11:27

In the middle of the exhibition hall, visitors to the grand fair in Geneva could immerse themselves, in the underground of science, visiting the large LHC accelerator and one of its experiments. (Image: Julien Ordan, Maximilien Brice/CERN)

In between a fondue tasting and trying out a reclining armchair, visitors to Geneva’s big annual fair, the Automnales had the chance to immerse themselves for a few minutes in the world of fundamental science. CERN was the guest of honour at this unmissable regional event, held from 10 to 19 November. It was an excellent opportunity for us to go out and meet members of the public who might never have thought to visit a research laboratory otherwise. 

The CERN stand was designed to resemble a particle collision, at the centre of which visitors could take a virtual-reality tour. Around this workshops, film screenings, games and interactive screens were offered. CERN volunteers explained the research, its technologies and its applications to visitors. (Image: Maximilien Brice, Julien Ordan)

Some 172 volunteers from CERN presented science in an entertaining way to visitors at the Automnales. Some 145 000 people attended the fair, and most of them stopped at the CERN stand, which was in a prime location in the middle of the hall.

The stand covered 1000 square meters and was designed to resemble a particle collision. Guests young and old had the chance to take a virtual-reality tour of the Large Hadron Collider (LHC) and one of its detectors, to learn how to conduct physics experiments using household objects, to marvel at shows about electricity and the cold, to play proton football, to programme robots and to travel through a tunnel taking them back in time to the Big Bang. Most importantly, they met CERN’s enthusiastic researchers, engineers, technicians and administrative employees, who were all delighted to share their passion for research. In short, ten days of enriching discoveries, meetings and exchanges.

For more information and pictures of the event, read this article.

For ten days, CERN was guest of honour at the Automnales, the annual trade fair in Geneva. (Video: Jacques Fichet/CERN)

 

CERN surprises Swiss expo visitors

Tue, 11/21/2017 - 11:27

In the middle of the exhibition hall, visitors to the grand fair in Geneva could immerse themselves, in the underground of science, visiting the large LHC accelerator and one of its experiments. (Image: Julien Ordan, Maximilien Brice/CERN)

In between a fondue tasting and trying out a reclining armchair, visitors to Geneva’s big annual fair, the Automnales had the chance to immerse themselves for a few minutes in the world of fundamental science. CERN was the guest of honour at this unmissable regional event, held from 10 to 19 November. It was an excellent opportunity for us to go out and meet members of the public who might never have thought to visit a research laboratory otherwise. 

The CERN stand was designed to resemble a particle collision, at the centre of which visitors could take a virtual-reality tour. Around this workshops, film screenings, games and interactive screens were offered. CERN volunteers explained the research, its technologies and its applications to visitors. (Image: Maximilien Brice, Julien Ordan)

Some 172 volunteers from CERN presented science in an entertaining way to visitors at the Automnales. Some 145 000 people attended the fair, and most of them stopped at the CERN stand, which was in a prime location in the middle of the hall.

The stand covered 1000 square meters and was designed to resemble a particle collision. Guests young and old had the chance to take a virtual-reality tour of the Large Hadron Collider (LHC) and one of its detectors, to learn how to conduct physics experiments using household objects, to marvel at shows about electricity and the cold, to play proton football, to programme robots and to travel through a tunnel taking them back in time to the Big Bang. Most importantly, they met CERN’s enthusiastic researchers, engineers, technicians and administrative employees, who were all delighted to share their passion for research. In short, ten days of enriching discoveries, meetings and exchanges.

For more information and pictures of the event, read this article.

For ten days, CERN was guest of honour at the Automnales, the annual trade fair in Geneva. (Video: Jacques Fichet/CERN)

 

50 years since iconic 'A Model of Leptons' published

Mon, 11/20/2017 - 10:29

This event shows the real tracks produced in the 1200 litre Gargamelle bubble chamber that provided the first confirmation of a neutral current interaction. (Image: CERN)

Today, 50 years ago, Steven Weinberg published the iconic paper A Model of Leptons, which determined the direction for high-energy particle physics research from them on. This paper lies at the core of the Standard Model, our most complete theory of how particles interact in our universe.

Just two pages long, Weinberg’s elegant and simply written theory was revolutionary at the time, yet was virtually ignored for many years. But now, it is cited at least three times a week.

Proving the validity of Weinberg’s theory inspired one of the biggest experimental science programmes ever seen and CERN has built major projects with these discoveries at their heart: the Gargamelle bubble chamber found the first evidence of the electroweak current in 1973; the Super Proton Synchrotron showed, in 1982, the first evidence of the W boson; and most recently the Large Hadron Collider, in 2012, confirmed the existence of the Higgs Boson.


Steven Weinberg visiting the ATLAS collaboration in 2009. (Image: Maximilien Brice/CERN)

Speaking to the CERN Courier Weinberg, now 84, describes what it’s like to see his work confirmed: “It’s what keeps you going as a theoretical physicist to hope that one of your squiggles will turn out to describe reality.” He received the Nobel Prize for this iconic, game-changing theory in 1979.

 

Read more about the original theory, and an interview with Steven Weinberg in this month’s CERN Courier

50 years since iconic 'A Model of Leptons' published

Mon, 11/20/2017 - 10:29

This event shows the real tracks produced in the 1200 litre Gargamelle bubble chamber that provided the first confirmation of a neutral current interaction. (Image: CERN)

Today, 50 years ago, Steven Weinberg published the iconic paper A Model of Leptons, which explains the profound link between mathematics and nature. This paper lies at the core of the Standard Model, our most complete theory of how particles interact in our universe.

Just two pages long, Weinberg’s elegant and simply written theory was revolutionary at the time, yet was virtually ignored for many years. But now, it is cited at least three times a week.

The paper uses the idea of symmetry – that everything in our universe has a corresponding mirror image – between particles called pions to build Weinberg’s theory of the fundamental forces.

From 1965 Weinberg had been building a mathematical structure and theorems based on this symmetry that explained why physicists had observed certain interactions between pions and nucleons and how pions behave when they are scattered from one another. This paved the way for a whole theory of hadronic physics at low energy.

“It’s what keeps you going as a theoretical physicist to hope that one of your squiggles will turn out to describe reality.”
Steven Weinberg, Nobel prize winner and author of A Model of Leptons

Physicists had been using the concept of symmetry since the 1930’s, but had not yet been able to unite the electromagnetic and weak forces. Uniting the two forces would bring physicists closer to a single theory describing how and why all the fundamental interactions in our universe occur. The mathmatics needed the particles carrying these two forces to be massless, but Weinberg and other physicists knew that if the particles really created these forces in nature, they had to be very heavy.

One day, as the 34-year-old Weinberg was driving his red Camero to work, he had a flash of insight – he had been looking for massless particles in the wrong place. He applied his theory to a rarely mentioned and often disregarded particle, the massive W boson, and paired it with a massless photon.  Theorists accounted for the mass of the W by introducing another unseen mechanism. This later became known as the Higgs mechanism, which calls for the existence of a Higgs boson.

Proving the validity of Weinberg’s theory inspired one of the biggest experimental science programmes ever seen and CERN has built major projects with these discoveries at their heart: the Gargamelle bubble chamber found the first evidence of the electroweak current in 1973; the Super Proton Synchrotron showed, in 1982, the first evidence of the W boson; and most recently the Large Hadron Collider, in 2012, confirmed the existence of the Higgs Boson.


Steven Weinberg visiting the ATLAS collaboration in 2009. (Image: Maximilien Brice/CERN)

Speaking to the CERN Courier Weinberg, now 84, describes what it’s like to see his work confirmed: “It’s what keeps you going as a theoretical physicist to hope that one of your squiggles will turn out to describe reality.” He received the Nobel Prize for this iconic, game-changing theory in 1979.

Half a century after this publication, it’s hard to find a theory that explains fundamental physics as clearly as Weinberg’s, which brought together all the different pieces of the puzzle and assembled them into one, very simple idea.

 

Read more about the original theory, and an interview with Steven Weinberg in this month’s CERN Courier

Record luminosity: well done LHC

Mon, 11/13/2017 - 14:56

View of the LHC tunnel. (Image : Maximilien Brice/CERN)

It’s the end of the road for the protons this year after a magnificent performance from the Large Hadron Collider (LHC). On Friday, the final beams of the 2017 proton run circulated in the LHC. The run ended, as it does every year, with a round up of the luminosity performance, the indicator by which the effectiveness of a collider is measured and on which the operators keep a constant eye.

The LHC has far exceeded its target for 2017. It has provided its two major experiments, ATLAS and CMS, with 50 inverse femtobarns of data, i.e. 5 billion million collisions. The inverse femtobarn (fb-1) is the unit used to measure integrated luminosity, or the cumulative number of potential collisions over a given period.

This result is all the more remarkable because the machine experts had to overcome a serious setback. A vacuum problem in the beam pipe of a magnet cell limited the number of bunches that could circulate in the machine. Several teams were brought in to find a solution. Notably, the arrangement of the bunches in the beams was changed. After a few weeks, luminosity started to increase again.

At the same time, over the course of the year, the operators have optimised the operating parameters. Using a new system put in place this year, they have notably reduced the size of the beams when they meet at the centre of the experiments. The more squeezed the beams, the more collisions occur each time they meet. Last year, the operators managed to obtain 40 collisions at each bunch crossing, with each bunch containing 100 billion particles. In 2017, up to 60 collisions were produced at each crossing.

Thanks to these improvements, the instantaneous luminosity record was smashed, reaching 2.06 x 1034cm-2s-1, or twice the nominal value. Instantaneous luminosity corresponds to the potential number of collisions per second.

The LHC will continue to operate for another two weeks for two special runs including a week for operation studies. The first special run will consist of carrying out proton collisions at 5.02 TeV (as opposed to the usual 13 TeV), the same energy as that planned for next year’s lead-ion runs. This will enable physicists to collect data with protons, which they will then be able to compare with the lead-ion data.

The second special run, at very low luminosity, will provide data for the TOTEM and ATLAS/ALFA experiments. These two experiments use detectors located on either side of two large LHC detectors: CMS in the case of TOTEM and ATLAS in the case of ATLAS/ALFA. They study interactions called elastic scattering, where two protons merely change direction slightly when they interact, rather than colliding. For these studies, the LHC makes the beams as wide as possible. What’s more, the energy will be limited to 450 GeV, i.e. the energy at which beams are injected from the accelerator complex into the LHC.

Finally, the operators will carry out a “machine development” campaign. Over a week, they will perform operating tests to improve the accelerator’s performance still further (it can never be too good) and begin to prepare the High-Luminosity LHC, which will take over from the LHC after 2025.

When these tests are over, the operators will stop the machine for the year-end technical shutdown. 

Graphs showing the integrated luminosity of the LHC in 2017. The unit is the inverse femtobarn. The green squares represent the achieved luminosity, while the blue line shows the planned luminosity. (Image: CERN)

Record luminosity: well done LHC

Mon, 11/13/2017 - 14:56

View of the LHC tunnel. (Image : Maximilien Brice/CERN)

It’s the end of the road for the protons this year after a magnificent performance from the Large Hadron Collider (LHC). On Friday, the final beams of the 2017 proton run circulated in the LHC. The run ended, as it does every year, with a round up of the luminosity performance, the indicator by which the effectiveness of a collider is measured and on which the operators keep a constant eye.

The LHC has far exceeded its target for 2017. It has provided its two major experiments, ATLAS and CMS, with 50 inverse femtobarns of data, i.e. 5 billion million million collisions. The inverse femtobarn (fb-1) is the unit used to measure integrated luminosity, or the cumulative number of potential collisions over a given period.

This result is all the more remarkable because the machine experts had to overcome a serious setback. A vacuum problem in the beam pipe of a magnet cell limited the number of bunches that could circulate in the machine. Several teams were brought in to find a solution. Notably, the arrangement of the bunches in the beams was changed. After a few weeks, luminosity started to increase again.

At the same time, over the course of the year, the operators have optimised the operating parameters. Using a new system put in place this year, they have notably reduced the size of the beams when they meet at the centre of the experiments. The more squeezed the beams, the more collisions occur each time they meet. Last year, the operators managed to obtain 40 collisions at each bunch crossing, with each bunch containing 100 billion particles. In 2017, up to 60 collisions were produced at each crossing.

Thanks to these improvements, the instantaneous luminosity record was smashed, reaching 2.06 x 1034cm-2s-1, or twice the nominal value. Instantaneous luminosity corresponds to the potential number of collisions per second.

The LHC will continue to operate for another two weeks for two special runs including a week for operation studies. The first special run will consist of carrying out proton collisions at 5.02 TeV (as opposed to the usual 13 TeV), the same energy as that planned for next year’s lead-ion runs. This will enable physicists to collect data with protons, which they will then be able to compare with the lead-ion data.

The second special run, at very low luminosity, will provide data for the TOTEM and ATLAS/ALFA experiments. These two experiments use detectors located on either side of two large LHC detectors: CMS in the case of TOTEM and ATLAS in the case of ATLAS/ALFA. They study interactions called elastic scattering, where two protons merely change direction slightly when they interact, rather than colliding. For these studies, the LHC makes the beams as wide as possible. What’s more, the energy will be limited to 450 GeV, i.e. the energy at which beams are injected from the accelerator complex into the LHC.

Finally, the operators will carry out a “machine development” campaign. Over a week, they will perform operating tests to improve the accelerator’s performance still further (it can never be too good) and begin to prepare the High-Luminosity LHC, which will take over from the LHC after 2025.

When these tests are over, the operators will stop the machine for the year-end technical shutdown. 

Graphs showing the integrated luminosity of the LHC in 2017. The unit is the inverse femtobarn. The green squares represent the achieved luminosity, while the blue line shows the planned luminosity. (Image: CERN)

Combatting cancer in challenging environments

Thu, 11/09/2017 - 15:59

World map showing access to radiotherapy treatment centres and the shortfall of more than 5000 radiotherapy machines in low- to middle-income countries. (Image: IAEA, AGaRT)

If you live in a low- or middle-income country, your chances of surviving cancer are significantly lower than if you live in a wealthier economy, and that’s largely due to the availability of radiation therapy.

A group of international experts in the fields of accelerator design, medical physics and oncology recently met at CERN to try to solve the technical problem of designing a robust linear accelerator (linac) that can be used in more challenging environments. 

Between 2015 and 2035, the number of cancer diagnoses worldwide is expected to increase by 10 million, with around 65% of those cases in poorer economies. 

It’s estimated that 12 600 new radiotherapy treatment machines will be needed to treat those patients. 

“We need to develop a machine that provides state-of-the-art radiation therapy in situations where the power supply is unreliable, the climate is harsh or communications are poor,” explains Manjit Dosanjh, senior advisor for CERN medical applications. “We need to avoid a linac of sub-standard quality that would not only provide lower-quality treatment but would be a disincentive for the recruitment and retention of high-quality staff.”

Limiting factors to the development and implementation of radiotherapy in lower-resourced nations don’t just include the cost of equipment and infrastructure, but also a shortage of trained personnel to properly calibrate and maintain the equipment and to deliver high-quality treatment. The plan is to design a medical accelerator that is affordable, easy to operate and maintain, and robust enough to be used in areas where these operational challenges might occur.

“I grew up in Australia, where the distances to hospitals can be vast, the climate can be harsh and local access to medical experts can quite literally be the difference between life and death,” explains accelerator physicist Suzie Sheehy from the University of Oxford and the Science and Technology Facilities Council (STFC). “In this project, the challenges in different environments will be extremely varied, but it seems obvious to me that those of us on the cutting-edge of research in particle accelerators should rise to the challenge of re-designing systems to make them more available to those who need them. I see this as a challenge and an opportunity to take my research into spaces where it is most needed.”

Jointly organised by CERN, the International Cancer Expert Corps (ICEC) and STFC, the workshop at CERN from 26 to 27 October was funded through the UK’s Global Challenges Research Fund, enabling key participants from Botswana, Ghana, Jordan, Nigeria and Tanzania to share their grass-roots perspectives. Understanding the in-country challenges will improve the effectiveness of the technology under design. Zubi Zubizaretta of the International Atomic Energy Agency (IAEA) also presented the results of the 2017 IAEA Radiation Therapy survey.

This workshop followed on from the inaugural workshop in November 2016, and a future ICEC workshop will look at the education and training requirements for the estimated 130 000 local staff (oncologists, medical physicists and technicians) who will be needed to operate the treatment machines and deliver patient care.

This ambitious project aims to have facilities and staff available to treat patients in low- and middle-income countries within 10 years.

Combatting cancer in challenging environments

Thu, 11/09/2017 - 15:59

World map showing access to radiotherapy treatment centres and the shortfall of more than 5000 radiotherapy machines in low- to middle-income countries. (Image: IAEA, AGaRT)

If you live in a low- or middle-income country, your chances of surviving cancer are significantly lower than if you live in a wealthier economy, and that’s largely due to the availability of radiation therapy.

A group of international experts in the fields of accelerator design, medical physics and oncology recently met at CERN to try to solve the technical problem of designing a robust linear accelerator (linac) that can be used in more challenging environments. 

Between 2015 and 2035, the number of cancer diagnoses worldwide is expected to increase by 10 million, with around 65% of those cases in poorer economies. 

It’s estimated that 12 600 new radiotherapy treatment machines will be needed to treat those patients. 

“We need to develop a machine that provides state-of-the-art radiation therapy in situations where the power supply is unreliable, the climate is harsh or communications are poor,” explains Manjit Dosanjh, senior advisor for CERN medical applications. “We need to avoid a linac of sub-standard quality that would not only provide lower-quality treatment but would be a disincentive for the recruitment and retention of high-quality staff.”

Limiting factors to the development and implementation of radiotherapy in lower-resourced nations don’t just include the cost of equipment and infrastructure, but also a shortage of trained personnel to properly calibrate and maintain the equipment and to deliver high-quality treatment. The plan is to design a medical accelerator that is affordable, easy to operate and maintain, and robust enough to be used in areas where these operational challenges might occur.

“I grew up in Australia, where the distances to hospitals can be vast, the climate can be harsh and local access to medical experts can quite literally be the difference between life and death,” explains accelerator physicist Suzie Sheehy from the University of Oxford and the Science and Technology Facilities Council (STFC). “In this project, the challenges in different environments will be extremely varied, but it seems obvious to me that those of us on the cutting-edge of research in particle accelerators should rise to the challenge of re-designing systems to make them more available to those who need them. I see this as a challenge and an opportunity to take my research into spaces where it is most needed.”

Jointly organised by CERN, the International Cancer Expert Corps (ICEC) and STFC, the workshop at CERN from 26 to 27 October was funded through the UK’s Global Challenges Research Fund, enabling key participants from Botswana, Ghana, Jordan, Nigeria and Tanzania to share their grass-roots perspectives. Understanding the in-country challenges will improve the effectiveness of the technology under design. Zubi Zubizaretta of the International Atomic Energy Agency (IAEA) also presented the results of the 2017 IAEA Radiation Therapy survey.

This workshop followed on from the inaugural workshop in November 2016, and a future ICEC workshop will look at the education and training requirements for the estimated 130 000 local staff (oncologists, medical physicists and technicians) who will be needed to operate the treatment machines and deliver patient care.

This ambitious project aims to have facilities and staff available to treat patients in low- and middle-income countries within 10 years.

Marie Skłodowska-Curie: more alive today than ever

Tue, 11/07/2017 - 08:14

Exactly 150 years ago, on 7 November 1867, Marie Skłodowska was born in Warsaw in Poland. A century and a half later, the name Marie Skłodowska-Curie is associated not only with this double-Nobel-prizewinning scientific luminary, but also with a whole community of European scientists: the Marie Skłodowska-Curie fellows.

Since the programme was introduced by the European Commission in 1990, the Marie Skłodowska-Curie fellowships have benefitted more than 100 000 scientists at all stages of their careers (from doctoral students to experienced researchers). Above all, the programme aims to promote international and interdisciplinary mobility and excellence in research across all fields.

Since 2004, the Marie Skłodowska-Curie programme has enabled more than 490 fellows to continue their studies at CERN, usually for a period of two years. At present, 134 participants in the programme are spread across various departments of the Laboratory.

The aim of the Marie Skłodowska-Curie programme fits perfectly with CERN’s training mission. Several hundred undergraduate, doctoral and post-doctoral students have already benefited from CERN’s exceptional scientific environment and the know-how of its researchers, and the Marie Skłodowska-Curie programme has played a key role in making this happen. No doubt Marie Skłodowska-Curie herself would be proud of this success.

 

______________________________________

The Marie Skłodowska-Curie programme through the eyes of its fellows

 

Alessandra Gnecchi has been a Marie Skłodowska-Curie fellow in CERN’s Theoretical Physics department since April 2017. She is currently working on black holes in supersymmetric theories.

Alessandra Gnecchi. (Image: Julien Ordan/CERN)

 

“Marie Curie was the first female role model of the modern scientific era – I have a particular attachment to her. I was a young girl in the 1990s and read the book "Madame Curie", which made me decide to become a scientist.

Today, the Marie Skłodowska-Curie Fellowship Programme allows scientists to study more than one research topic, which is very important. It demonstrates moreover that the recipient was able to write a challenging proposal. Because of these aspects, this fellowship could allow my career to become highly visible and productive, and it is up to me now to exploit this opportunity.”

 

 

 

Roberto Cardella. (Image: Julien Ordan/CERN)

Roberto Cardella has been a Marie Skłodowska-Curie fellow in CERN’s Experimental Physics department since September 2016. He is currently working on the upgrade for the inner tracker of the ATLAS experiment.

“My Marie Skłodowska-Curie Fellowship falls under an ITN (Innovative Training Network) called STREAM. In our consortium, there are currently 17 fellows, spread all over Europe, working on related topics. It is inspiring to work on an innovative topic and to collaborate with other students.

This programme is a great opportunity for my career. Being part of a training network has already allowed me to get in touch with many institutes all over Europe. I am learning a lot from my colleagues here at CERN and the periodic meetings with the other students and partners in STREAM allow me to broaden my view of this field.”

 

 

 

Anna Stakia. (Image: Sophia Bennett/CERN)

Anna Stakia has been a Marie Skłodowska-Curie fellow in CERN’s Experimental Physics department since May 2016. She is currently working on New Physics searches and Machine Learning.

“Marie Skłodowska-Curie is without a doubt one of the most eminent figures in physics, and in science in general. I feel honoured to be part of a programme that carries her name and I am personally incredibly inspired by it.

To me, the strongest aspect of the Marie Skłodowska-Curie Programme is that it offers a broad variety of sub-academic and training options through which a fellow can navigate. In this way, any strict academic barriers are overcome. At the same time, the mobility opportunities enhance the fruitful interaction of students not only with researchers and working environments in foreign countries, but also among themselves, thus creating a fertile field for collaboration, which expands their research horizons, accelerates their progress and boosts their career potential.”

 

______________________________________

Today, to mark 150 years since the birth of Marie Skłodowska-Curie, CERN, the University of Liverpool and the Ludwig Maximilian University of Munich are organising a series of events for the scientific community and the general public. For more information, visit the event website.

From 3 pm (CET), watch the presentations at the University of Liverpool and at CERN.

To go further, read the article published in July on Marie Curie's granddaughter's visit to CERN.

Marie Skłodowska-Curie: more alive today than ever!

Tue, 11/07/2017 - 08:14

Exactly 150 years ago, on 7 November 1867, Marie Skłodowska was born in Warsaw in Poland. A century and a half later, the name Marie Skłodowska-Curie is associated not only with this double-Nobel-prizewinning scientific luminary, but also with a whole community of European scientists: the Marie Skłodowska-Curie fellows.

Since the programme was introduced by the European Commission in 1990, the Marie Skłodowska-Curie fellowships have benefitted more than 100 000 scientists at all stages of their careers (from doctoral students to experienced researchers). Above all, the programme aims to promote international and interdisciplinary mobility and excellence in research across all fields.

Since 2004, the Marie Skłodowska-Curie programme has enabled more than 490 fellows to continue their studies at CERN, usually for a period of two years. At present, 134 participants in the programme are spread across various departments of the Laboratory.

The aim of the Marie Skłodowska-Curie programme fits perfectly with CERN’s training mission. Several hundred undergraduate, doctoral and post-doctoral students have already benefited from CERN’s exceptional scientific environment and the know-how of its researchers, and the Marie Skłodowska-Curie programme has played a key role in making this happen. No doubt Marie Skłodowska-Curie herself would be proud of this success.

 

______________________________________

The Marie Skłodowska-Curie programme through the eyes of its fellows

 

Alessandra Gnecchi has been a Marie Skłodowska-Curie fellow in CERN’s Theoretical Physics department since April 2017. She is currently working on black holes in supersymmetric theories.

Alessandra Gnecchi. (Image: Julien Ordan/CERN)

 

“Marie Curie was the first female role model of the modern scientific era – I have a particular attachment to her. I was a young girl in the 1990s and read the book "Madame Curie", which made me decide to become a scientist.

Today, the Marie Skłodowska-Curie Fellowship Programme allows scientists to study more than one research topic, which is very important. It demonstrates moreover that the recipient was able to write a challenging proposal. Because of these aspects, this fellowship could allow my career to become highly visible and productive, and it is up to me now to exploit this opportunity.”

 

 

 

Roberto Cardella. (Image: Julien Ordan/CERN)

Roberto Cardella has been a Marie Skłodowska-Curie fellow in CERN’s Experimental Physics department since September 2016. He is currently working on the upgrade for the inner tracker of the ATLAS experiment.

“My Marie Skłodowska-Curie Fellowship falls under an ITN (Innovation Training Network) called STREAM. In our consortium, there are currently 17 fellows, spread all over Europe, working on related topics. It is inspiring to work on an innovative topic and to collaborate with other students.

This programme is a great opportunity for my career. Being part of a training network has already allowed me to get in touch with many institutes all over Europe. I am learning a lot from my colleagues here at CERN and the periodic meetings with the other students and partners in STREAM allow me to broaden my view of this field.”

 

 

 

Anna Stakia. (Image: Sophia Bennett/CERN)

Anna Stakia has been a Marie Skłodowska-Curie fellow in CERN’s Experimental Physics department since May 2016. She is currently working on New Physics searches and Machine Learning.

“Marie Skłodowska-Curie is without a doubt one of the most eminent figures in physics, and in science in general. I feel honoured to be part of a programme that carries her name and I am personally incredibly inspired by it.

To me, the strongest aspect of the Marie Skłodowska-Curie Programme is that it offers a broad variety of sub-academic and training options through which a fellow can navigate. In this way, any strict academic barriers are overcome. At the same time, the mobility opportunities enhance the fruitful interaction of students not only with researchers and working environments in foreign countries, but also among themselves, thus creating a fertile field for collaboration, which expands their research horizons, accelerates their progress and boosts their career potential.”

 

______________________________________

Today, to mark 150 years since the birth of Marie Skłodowska-Curie, CERN, the University of Liverpool and the Ludwig Maximilian University of Munich are organising a series of events for the scientific community and the general public. For more information, visit the event website.

From 3 pm (CET), watch the presentations at the University of Liverpool and at CERN.

To go further, read the article published in July on Marie Curie's granddaughter's visit to CERN.

How much does a kilogram weigh?

Thu, 11/02/2017 - 15:49

The National Institute of Standards and Technology (NIST)-4 Kibble balance measured Planck's constant to within 13 parts per billion in 2017, accurate enough to assist with the redefinition of the kilogram. (Image: J. L. Lee/NIST)

The Kilogram doesn’t weigh a kilogram any more. This sad news was announced during a seminar at CERN on Thursday, 26 October by Professor Klaus von Klitzing, who was awarded the 1985 Nobel Prize in Physics for the discovery of the quantised Hall effect. “We are about to witness a revolutionary change in the way the kilogram is defined,” he declared.  

Together with six other units – metre, second, ampere, kelvin, mole, and candela – the kilogram, a unit of mass, is part of the International System of Units (SI) that is used as a basis to express every measurable object or phenomenon in nature in numbers. This unit’s current definition is based on a small platinum and iridium cylinder, known as “le grand K”, whose mass is exactly one kilogram. The cylinder was crafted in 1889 and, since then, has been kept safe under three glass bell jars in a high-security vault on the outskirts of Paris. There is one problem: the current standard kilogram is losing weight. About 50 micrograms, at the latest check. Enough to be different from its once-identical copies stored in laboratories around the world. 

To solve this weight(y) problem, scientists have been looking for a new definition of the kilogram.

At the quadrennial General Conference on Weights and Measures in 2014, the scientific metrology community formally agreed to redefine the kilogram in terms of the Planck constant (h), a quantum-mechanical quantity relating a particle’s energy to its frequency, and, through Einstein’s equation E = mc2, to its mass. Planck’s constant is one of the fundamental numbers of our universe, a quantity fixed universally in nature, such as the speed of light or the electric charge of a proton.

Planck’s constant will be assigned an exact fixed value based on the best measurements obtained worldwide. The kilogram will be redefined through the relationship between Planck’s constant and mass.

Replica of the national prototype kilogram standard no. K20 kept by the US government National Institute of Standards and Technology (NIST), Bethesda, Maryland. (Image: National Institute of Standards and Technology)

“There’s nothing to be worried about,” says Klaus von Klitzing. “The new kilogram will be defined in such a way that (nearly) nothing will change in our daily life. It won’t make the kilogram more precise either, it will just make it more stable and more universal.”

However, the redefinition process is not that simple. The International Committee for Weights and Measures, the governing body responsible for ensuring international agreement on measurements, has imposed strict requirements on the procedure to follow: three independent experiments measuring the Planck constant must agree on the derived value of the kilogram with uncertainties below 50 parts per billion, and at least one must achieve an uncertainty below 20 parts per billion. Fifty parts per billion in this case equals approximately 50 micrograms – about the weight of an eyelash.

Two types of experiment have proved able to link the Planck constant to mass with such extraordinary precision. One method, led by an international team known as the Avogadro Project, entails counting the atoms in a silicon-28 sphere that weighs the same as the reference kilogram. The second method involves a sort of scale known as a watt (or Kibble) balance. Here, electromagnetic forces are counterbalanced by a test mass calibrated according to the reference kilogram.

And that’s where the important discovery made by Klaus von Klitzing in 1980, which earned him the Nobel Prize in Physics, comes into play. In order to get extremely precise measurements of the current and voltage making up the electromagnetic forces in the watt balance, scientists use two different quantum-electrical universal constants. One of these is the von Klitzing constant, which is known with extreme precision, and can in turn be defined in terms of the Planck constant and the charge of the electron. The von Klitzing constant describes how resistance is quantised in a phenomenon called the “quantum Hall effect”, a quantum-mechanical phenomenon observed when electrons are confined in an extra-thin metallic layer subjected to low temperatures and strong magnetic fields.

This is truly a big revolution,” von Klitzing says. “In fact, it has been dubbed the biggest revolution in metrology since the French Revolution, when the first global system of units was introduced by the French Academy of Sciences.”

CERN is playing its part in this revolution. The Laboratory participated in a metrology project launched by the Swiss Metrology Office (METAS) to build a watt balance, which will be used to disseminate the definition of the new kilogram through extremely precise measurements of the Planck constant. CERN provided a crucial element of the watt balance: the magnetic circuit, which is needed to generate the electromagnetic forces balanced by the test mass. The magnet needs to be extremely stable during the measurement and provide a very homogenous magnetic field. 

Combien pèse un kilo ?

Thu, 11/02/2017 - 15:49

La balance du watt NIST-4 du National Institute of Standards and Technology a mesuré la constante de Planck avec une précision de 13 parties par milliard, précision suffisante pour participer à la redéfinition du kilogramme. (Image : J. L. Lee/NIST)

Le Kilogramme ne pèse plus un kilogramme. Cette nouvelle déroutante a été annoncée lors d’un séminaire au CERN, jeudi 26 octobre, par Klaus von Klitzing, qui a reçu en 1985 le prix Nobel de physique pour la découverte de l’effet Hall quantique. « Nous allons être témoins d’un changement révolutionnaire dans la manière de définir le kilogramme », a-t-il indiqué. 

Le kilogramme, unité de masse, fait partie avec six autres unités (le mètre, la seconde, l’ampère, le kelvin, la mole et la candela) du Système international d’unités (SI), qui sert de référence pour exprimer par des nombres tout objet ou phénomène naturel mesurable. La définition actuelle du kilogramme se base sur un petit cylindre en alliage de platine et d’iridium, surnommé le « grand K » et pesant exactement un kilogramme. Ce cylindre a été fabriqué en 1889 et il est depuis gardé en sûreté, protégé par trois cloches de verre dans un coffre-fort de haute sécurité dans la banlieue de Paris. Mais il y a un problème : l’étalon du kilogramme perd du poids. Il s’agirait, d’après la dernière vérification, d’environ 50 microgrammes. Ces 50 microgrammes suffisent à ce que l’étalon ne soit plus, comme c’était initialement le cas, identique à ses copies, conservées dans des laboratoires du monde entier. 

Pour résoudre ce problème de poids, les scientifiques ont cherché une nouvelle manière de définir le kilogramme.

Lors de la Conférence générale quadriennale des poids et mesures de 2014, la communauté scientifique de la métrologie a décidé officiellement de redéfinir le kilogramme en fonction de la constante de Planck (h), quantité issue de la mécanique quantique qui met en relation l’énergie d’une particule avec sa fréquence, mais aussi, grâce à l’équation d’Einstein E = mc2, avec sa masse. La constante de Planck est l’un des nombres fondamentaux de notre Univers, une quantité dont la valeur reste universellement fixe dans la nature, comme la vitesse de la lumière ou la charge électrique d’un proton. 

Le K 20, réplique de l’étalon du kilogramme et prototype national, conservé par le gouvernement des États-Unis au NIST à Bethesda, Maryland. (Image: NIST)

Une valeur fixe exacte sera déterminée pour la constante de Planck, sur la base des meilleures mesures obtenues dans le monde entier. Le kilogramme sera ainsi redéfini grâce à la relation existant entre la constante de Planck et la masse.

« Il n’y a pas lieu de s’inquiéter, assure Klaus von Klitzing. Le kilogramme sera redéfini d’une manière qui ne changera (presque) rien dans notre quotidien. Sa valeur ne gagnera pas en précision ; elle deviendra néanmoins plus stable et plus universelle. »

Le processus de redéfinition n’est pourtant pas aussi simple qu’il n’y paraît. Le Bureau international des poids et mesures, organe responsable de la gestion des accords internationaux sur les mesures, a imposé des exigences strictes pour la procédure à suivre : trois expériences indépendantes mesurant la constante de Planck devront se mettre d’accord sur la valeur qui en sera dérivée pour le kilogramme, avec une marge d’incertitude inférieure à 50 parties par milliard (ppb), et à 20 ppb pour l’une d’entre elles au moins. 50 parties par milliard équivaut, dans le cas présent, à environ 50 microgrammes, soit à peu près le poids d’un cil.

Deux types d’expériences se sont révélées capables de lier la constante de Planck à la masse avec une précision aussi exceptionnelle. Une des méthodes, pratiquée par une équipe internationale connue sous le nom Avogadro Project, consiste à compter les atomes contenus dans une sphère de silicone 28 ayant exactement le même poids que l’étalon du kilogramme. La deuxième méthode requiert une balance d’un type particulier, appelée balance du watt. Elle fonctionne en équilibrant des forces électromagnétiques avec une masse, la masse utilisée pour ce test étant calibrée pour correspondre exactement à l’étalon du kilogramme.

C’est là qu’entre en jeu la découverte majeure faite par Klaus von Klitzing en 1980, qui lui a valu le prix Nobel de physique. Pour obtenir des mesures de très haute précision du courant et de la tension des forces électromagnétiques se trouvant dans la balance du watt, les scientifiques utilisent deux constantes universelles. La première est la constante de von Klitzing, qui est connue avec une extrême précision et qui peut à son tour être définie en fonction de la constante de Planck et de la charge de l’électron. La constante de von Klitzing décrit la manière dont la résistance est quantifiée dans le cadre d’un phénomène de mécanique quantique appelé « effet Hall quantique », observé lorsque des électrons sont confinés dans une couche de métal extrêmement fine soumise à des températures basses et à de forts champs magnétiques.

« Il s’agit vraiment d’une révolution majeure, conclut Klaus von Klitzing. En fait, on a même dit qu’il s’agissait de la plus grande révolution, dans la métrologie, depuis la Révolution française, quand le premier système international d’unités avait été introduit par l’Académie française des sciences. »

Le CERN participe à cette révolution : le Laboratoire a pris part à un projet de métrologie, initié par l’institut suisse de métrologie METAS, visant à construire une balance du watt qui jouera un rôle dans l’harmonisation de la nouvelle définition du kilogramme en fournissant des mesures extrêmement précises de la constante de Planck. Le CERN a fourni un élément essentiel de la balance du watt : le circuit magnétique nécessaire pour créer les forces électromagnétiques qui seront équilibrées par la masse de test. L’aimant doit être extrêmement stable pendant la mesure, et fournir un champ magnétique très homogène. 

Pages