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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)

LHC reaches 2017 targets ahead of schedule

Mon, 10/30/2017 - 16:59

Trillions of protons race around the LHC’s 27km ring in opposite directions more than 11,000 times a second, travelling at 99.9999991 per cent the speed of light. (Image: Max Brice and Julien Ordan/CERN)

Today, CERN Control Centre operators announced good news, the Large Hadron Collider (LHC) has successfully met its production target for 2017, delivering more than 45 inverse femtobarns* to the experiments.

This achievement was all the more impressive as it was ahead of schedule. The LHC still has 19 more days of proton collisions, continuing to provide physics data to the experiments. Yet earlier this year it looked unlikely that this target would be achieved. An issue had developed with a small group of magnets known as 16L2 that was affecting machine performance. Then, early September, thanks to effective and creative collaboration between different teams around CERN, alternative ways to deal with the technical issue were developed that made the LHC and its injector chain reach top performances again. In addition, by the end of September, the 2017 production run was shortened by advancing special runs planned for 2018 to 2017, putting yet more pressure on the operators to deliver in a smaller timeframe. 

The LHC has outperformed its target for 2017, delivering more collisions than expected to LHC experiments.

None-the-less with the target met, as well as another recent milestone of reaching twice the design luminosity, the LHC has once again shown its excellence. That being said, physicists are already looking to upgrades tens of years in the future and the physics potential that they bring. Today at CERN, scientists are gathering to begin a three-day workshop to review, extend and further refine understanding of the physics potential of the High Luminosity LHC – the planned upgrade of the LHC – and even beyond.

In the more immediate future, once the main proton physics run end this year, the LHC will have 15 days of special runs plus machine development before its winter shutdown begins on 11 December. At that point, the “Year-end technical stop” (YETS) will be used to help consolidate and improve the machine, ahead of its restart in spring 2018.

 

* The inverse femtobarn (fb-1) is the unit of measurement for integrated luminosity, indicating the cumulative number of potential collisions. One inverse femtobarn corresponds to around 80 million million collisions.

For one day only LHC collides xenon beams

Thu, 10/12/2017 - 22:35

One of the xenon ion collisions recorded by the ALICE detector. (Image: ALICE/CERN)

Today, the LHC is getting a taste of something unusual. For eight hours, the Large Hadron Collider is accelerating and colliding xenon nuclei, allowing the large LHC experiments, ATLAS, ALICE, CMS and LHCb, to record xenon collisions for the first time.

Xenon is a noble gas, present in miniscule quantities in the atmosphere. Its atoms consist of 54 protons and between 70 and 80 neutrons, depending on the isotope. The xenon collisions in the LHC (of atoms with 54 protons and 75 neutrons) are therefore similar to the heavy-ion collisions that are regularly carried out at the LHC. Normally, lead nuclei, which have a much greater mass, are used. “But a run with xenon nuclei was planned for the NA61/SHINE fixed-target experiment at the SPS (Super Proton Synchrotron),” explains Reyes Alemany Fernandez, who is in charge of heavy-ion runs. “We are therefore taking the opportunity for a short run with xenon at the LHC.

It’s a unique opportunity both to explore the LHC’s capabilities with a new type of beam and to obtain new physics results,” says John Jowett, the physicist in charge of heavy-ion beams at the LHC.

And who knows? Maybe this unprecedented run will lead to some surprising discoveries. “The experiments will conduct the same kind of analyses with xenon ions as they do with lead ions, but, because the xenon nuclei have less mass, the geometry of the collision is different,” explains Jamie Boyd, LHC programme coordinator, who is responsible for liaison between the LHC machine and experiment teams. Heavy-ion collisions allow physicists to study quark-gluon plasma, a state of matter that is thought to have briefly existed just after the Big Bang. In this extremely dense and hot primordial soup, quarks and gluons moved around freely, without being confined by the strong force of protons and neutrons, as they are in our Universe today.

Some of the teams who contributed to the xenon run, in the CERN control centre. (Image: Jules Ordan/CERN)

Switching from protons to xenon isn’t a piece of cake, however. A team has been preparing the accelerator complex for the xenon run since the start of the year. Atoms of the gas are accelerated and stripped of their 54 electrons in four successive accelerators before being launched into the LHC. “The number of bunches and the revolution frequency varies a lot between protons and xenon nuclei,” explains Reyes Alemany Fernandez. “One of the difficulties is adjusting and synchronising the accelerators’ radiofrequency systems.”

After the xenon run in the LHC lasting a few hours, xenon nuclei will continue to circulate in the accelerator complex, but only as far as the SPS. For eight weeks, the SPS will supply xenon ions to the NA61/SHINE experiment, which is also studying quark-gluon plasma, but whose analyses will complement those carried out by the LHC experiments. More specifically, NA61/SHINE is interested in the deconfinement point, a collision-energy threshold above which the creation of quark-gluon plasma would be possible. NA61/SHINE is thus systematically testing many collision energies using ions of different masses. After lead, beryllium and argon, it’s now xenon’s turn to take the stage.

A chart showing different types of stable nuclei, with their atomic number, i.e. the number of protons, Z, shown on the horizontal axis and the number of neutrons, N, shown on the vertical axis. The three types already accelerated in the LHC, i.e. protons (hydrogen), lead nuclei and xenon nuclei, are shown in red with their mass number, A (N + Z).

 

For one day only LHC collides xenon beams

Thu, 10/12/2017 - 22:35

The team working on the ion run in the CERN control centre as the xenon run begins. (Image: Jules Ordan/CERN)

Today, the LHC is getting a taste of something unusual. For eight hours, the Large Hadron Collider is accelerating and colliding xenon nuclei, allowing the large LHC experiments, ATLAS, ALICE, CMS and LHCb, to record xenon collisions for the first time.

Xenon is a noble gas, present in miniscule quantities in the atmosphere. Its atoms consist of 54 protons and between 70 and 80 neutrons, depending on the isotope. The xenon collisions in the LHC (of atoms with 54 protons and 75 neutrons) are therefore similar to the heavy-ion collisions that are regularly carried out at the LHC. Normally, lead nuclei, which have a much greater mass, are used. “But a run with xenon nuclei was planned for the NA61/SHINE fixed-target experiment at the SPS (Super Proton Synchrotron),” explains Reyes Alemany Fernandez, who is in charge of heavy-ion runs. “We are therefore taking the opportunity for a short run with xenon at the LHC.

It’s a unique opportunity both to explore the LHC’s capabilities with a new type of beam and to obtain new physics results,” says John Jowett, the physicist in charge of heavy-ion beams at the LHC.

And who knows? Maybe this unprecedented run will lead to some surprising discoveries. “The experiments will conduct the same kind of analyses with xenon ions as they do with lead ions, but, because the xenon nuclei have less mass, the geometry of the collision is different,” explains Jamie Boyd, LHC programme coordinator, who is responsible for liaison between the LHC machine and experiment teams. Heavy-ion collisions allow physicists to study quark-gluon plasma, a state of matter that is thought to have briefly existed just after the Big Bang. In this extremely dense and hot primordial soup, quarks and gluons moved around freely, without being confined by the strong force of protons and neutrons, as they are in our Universe today.

 

The LHC screen during the xenon-ion run. (Image: CERN)

Switching from protons to xenon isn’t a piece of cake, however. A team has been preparing the accelerator complex for the xenon run since the start of the year. Atoms of the gas are accelerated and stripped of their 54 electrons in four successive accelerators before being launched into the LHC. “The number of bunches and the revolution frequency varies a lot between protons and xenon nuclei,” explains Reyes Alemany Fernandez. “One of the difficulties is adjusting and synchronising the accelerators’ radiofrequency systems.”

After the xenon run in the LHC lasting a few hours, xenon nuclei will continue to circulate in the accelerator complex, but only as far as the SPS. For eight weeks, the SPS will supply xenon ions to the NA61/SHINE experiment, which is also studying quark-gluon plasma, but whose analyses will complement those carried out by the LHC experiments. More specifically, NA61/SHINE is interested in the deconfinement point, a collision-energy threshold above which the creation of quark-gluon plasma would be possible. NA61/SHINE is thus systematically testing many collision energies using ions of different masses. After lead, beryllium and argon, it’s now xenon’s turn to take the stage.

A chart showing different types of stable nuclei, with their atomic number, i.e. the number of protons, Z, shown on the horizontal axis and the number of neutrons, N, shown on the vertical axis. The three types already accelerated in the LHC, i.e. protons (hydrogen), lead nuclei and xenon nuclei, are shown in red with their mass number, A (N + Z).

 

LHC rocks the seesaw model

Tue, 09/26/2017 - 11:06

Members of the CMS collaboration removing the preshower from ECAL detector in the CMS cavern. (Image: M Brice/CERN)

For most of us, seesaws are the stuff of childhood memories. For theoretical physicists, they could explain one of the biggest mysteries in the field: why are neutrinos so incredibly light? 
Experiments at CERN’s Large Hadron Collider have now put the unlikely sounding “seesaw model” through one of its most stringent tests. 

Discovered 60 years ago, the neutrino was long thought to weigh nothing at all. But experiments in the late 1990s showed that neutrinos change type as they travel, implying that they have a small but non-zero mass. The mystery is why their masses are so small and yet not zero, as assumed by the Standard Model of particle physics. Most other elementary particles acquire their masses by interacting with Higgs bosons: the stronger the interaction the heavier the particle. But many physicists think it a stretch – “unnatural” even – that the Higgs boson interacts so feebly with the neutrino as to leave it at least a million times lighter than the already waif-like electron. 

The seesaw model, dreamed up in the 1980s, is an abstract ratio that connects normal neutrinos to an unseen breed of super-heavy particles with weird properties: the neutrinos we know on one end are pivoted up by heavier particles on the other end of the “seesaw”. Were these mathematical gymnastics shown to be responsible for the neutrino’s tiny mass, it would lead physicists to a rich landscape of new particles and phenomena beyond the Standard Model, perhaps even a unified theory of the fundamental forces.

Researchers on the CMS experiment have been searching for signs of the neutrino’s weightier partners among billions of proton-proton collisions produced by the LHC at an energy of 13 TeV. Specifically, they tested a version of the seesaw mechanism (called Type-III) that involves a triplet of two heavy charged particles and an additional neutral particle, which would reveal themselves in the CMS detector via their decays into more familiar objects such as Higgs bosons and electrons. Studies at the LHC have also probed the original Type-I seesaw mechanism, which requires a heavy “sterile” neutrino that does not interact at all with known matter.Such particles are the quarry of several dedicated neutrino experiments worldwide.

Building on previous search results obtained by CMS and its sister experiment ATLAS at lower collision energies, CMS reports no sign of heavy charged particles associated with the seesaw model and has ruled out their existence below a mass of 840 GeV. According to the team, these are the strongest constraints to date on the mass of Type-III seesaw particles. “We have now looked at all 27 relevant decay channels in a single multimodal analysis,” explains CMS member Sunil Somalwar from Rutgers University in the US. “With the seesaw, the higher you set the masses of the new particles the smaller the neutrino mass – which is good. We are now getting into seesaw exclusions at the LHC.” 

CMS and ATLAS will subject the seesaw, and many other models of physics beyond the Standard Model, to further scrutiny as the LHC continues to amass data at a record-breaking energy of 13 TeV.  

 

Ref: The CMS result is described in a preprint (arXiv:1708.07962) and has been submitted to the journal Physical Review Letters.

Immerse yourself into the CMS detector and observe the installation of the new Pixel Tracker from within the underground experimental cavern with this interactive 360º photograph. (Image: Max Brice/CERN)

LHC rocks the seesaw model

Tue, 09/26/2017 - 11:06

Members of the CMS collaboration removing the preshower from ECAL detector in the CMS cavern. (Image: M Brice/CERN)

For most of us, seesaws are the stuff of childhood memories. For theoretical physicists, they could explain one of the biggest mysteries in the field: why are neutrinos so incredibly light? 
Experiments at CERN’s Large Hadron Collider have now put the unlikely sounding “seesaw model” through one of its most stringent tests. 

Discovered 60 years ago, the neutrino was long thought to weigh nothing at all. But experiments in the late 1990s showed that neutrinos change type as they travel, implying that they have a small but non-zero mass. The mystery is why their masses are so small and yet not zero, as assumed by the Standard Model of particle physics. Most other elementary particles acquire their masses by interacting with Higgs bosons: the stronger the interaction the heavier the particle. But many physicists think it a stretch – “unnatural” even – that the Higgs boson interacts so feebly with the neutrino as to leave it at least a million times lighter than the already waif-like electron. 

The seesaw model, dreamed up in the 1980s, is an abstract ratio that connects normal neutrinos to an unseen breed of super-heavy particles with weird properties: the neutrinos we know on one end are pivoted up by heavier particles on the other end of the “seesaw”. Were these mathematical gymnastics shown to be responsible for the neutrino’s tiny mass, it would lead physicists to a rich landscape of new particles and phenomena beyond the Standard Model, perhaps even a unified theory of the fundamental forces.

Researchers on the CMS experiment have been searching for signs of the neutrino’s weightier partners among billions of proton-proton collisions produced by the LHC at an energy of 13 TeV. Specifically, they tested a version of the seesaw mechanism (called Type-III) that involves a triplet of two heavy charged particles and an additional neutral particle, which would reveal themselves in the CMS detector via their decays into more familiar objects such as Higgs bosons and electrons. Studies at the LHC have also probed the original Type-I seesaw mechanism, which requires a heavy “sterile” neutrino that does not interact at all with known matter.Such particles are the quarry of several dedicated neutrino experiments worldwide.

Building on previous search results obtained by CMS and its sister experiment ATLAS at lower collision energies, CMS reports no sign of heavy charged particles associated with the seesaw model and has ruled out their existence below a mass of 840 GeV. According to the team, these are the strongest constraints to date on the mass of Type-III seesaw particles. “We have now looked at all 27 relevant decay channels in a single multimodal analysis,” explains CMS member Sunil Somalwar from Rutgers University in the US. “With the seesaw, the higher you set the masses of the new particles the smaller the neutrino mass – which is good. We are now getting into seesaw exclusions at the LHC.” 

CMS and ATLAS will subject the seesaw, and many other models of physics beyond the Standard Model, to further scrutiny as the LHC continues to amass data at a record-breaking energy of 13 TeV.  

 

Immerse yourself into the CMS detector and observe the installation of the new Pixel Tracker from within the underground experimental cavern with this interactive 360º photograph. (Image: Max Brice/CERN)

TED-Ed: Emptying the vacuum

Tue, 09/12/2017 - 17:16

Could we create a perfect vacuum? In a universe filled with matter and energy, we often think of deepest outer space as a vacuum, empty of everything. But it is far from it, with a multitude of particles and electromagnetic radiation zooming through it. This new animation, made in collaboration with TED-Ed, explores why CERN’s accelerators need to be one of the emptiest spaces in the universe and asks if there is such a thing as totally empty space. 

Read more about the content in this animation on the TED-Ed website

 

TED-Ed: Emptying the vacuum

Tue, 09/12/2017 - 17:16

Could we create a perfect vacuum? In a universe filled with matter and energy, we often think of deepest outer space as a vacuum, empty of everything. But it is far from it, with a multitude of particles and electromagnetic radiation zooming through it. This new animation, made in collaboration with TED-Ed, explores why CERN’s accelerators need to be one of the emptiest spaces in the universe and asks if there is such a thing as totally empty space. 

Read more about the content in this animation on the TED-Ed website