This image shows a simulation of the electron clouds development when the proton beam passes through the vacuum chamber. (Image: CERN)
Protons are jostling for space in the Large Hadron Collider. Since the start of the physics run on 23 May, the operators of the huge accelerator have been increasing the intensity of the beams, injecting more and more protons in order to increase the number of collisions.
“Trains” of proton bunches have been circulating in the machine for the past week. Consisting of up to 288 bunches, each containing more than 100 billion protons, the trains are formed by the accelerator chain and then sent into the large ring. They are then accelerated to a speed close to that of light for around twenty minutes, before they collide with each other in the centre of each experiment. Recently, 600 bunches have been circulating in each direction. The aim is to reach 2500 bunches in each beam within a few weeks.
To achieve this, the machine specialists must first improve the surface conditions of the vacuum chambers in which the protons circulate. Obtaining the best possible vacuum is an essential prerequisite to make an accelerator work. Molecules remaining in the vacuum chamber are obstacles to the circulation of the protons – it is like sending Formula 1 cars around a track full of parked cars. Hence, before starting up the accelerator, the vacuum specialists pump the air out of the beam pipes, obtaining a high-quality vacuum, almost as good as on the surface of the moon (10-10 or even 10-11 millibar). This is enough to allow the circulation of a few hundred proton bunches, but beyond that, things get harder.
Despite the ultra-high vacuum, residual gas molecules and electrons remain trapped on the walls of the vacuum chambers. When the beam circulates, these electrons are liberated from the surface of the walls due to the impact of lost particles or photons emitted by the LHC proton beams. They are accelerated by the beam’s electrical field and hit the walls on the opposite side of the chamber, detaching trapped molecules and freeing more electrons. If the number of liberated electrons is larger than the number of impacting electrons, it may initiate an avalanche of electrons, which will destabilise the beam. This phenomenon, known as the “electron cloud”, is amplified by the large number of proton bunches and the short distance between the bunches in the beam.
To mitigate the impact of these clouds, the vacuum chamber can be conditioned with the beam itself. Increasing the number of circulating bunches frees as many molecules of gas as can be sustained and causes a massive release of electron clouds. Experience has shown that, once this operation, called "scrubbing", has been carried out, the production rate of gas molecules and electrons progressively falls. This allows the beam intensity to be increased stepwise until the LHC can be filled completely.
So it’s time for spring cleaning at the LHC. For five days, starting today, the LHC operators will carry out scrubbing of the vacuum chambers with beam. The physics run will take a short break, starting again in much better conditions mid-June.
An image of a proton–proton collision taken in the LHCb detector on 23 May. (Image: LHCb/CERN)
Physics at the LHC has kicked off for another season. Today, the Large Hadron Collider shifted up a gear, allowing the experiments to start taking data for the first time in 2017. Operations are starting gradually, with just a few proton bunches per beam. The operators who control the most powerful collider in the world will gradually increase the number of bunches circulating and will also reduce the size of the beams at the interaction points. In a few weeks’ time, over a billion collisions will be produced every second at the heart of the experiments.
Last year, the LHC produced an impressive amount of data, no fewer than 6.5 million billion collisions, representing an integrated luminosity over the course of the year of almost 40 inverse femtobarns. Luminosity, which corresponds to the number of potential collisions per surface unit in a given time period, is a crucial indicator of an accelerator’s performance. In 2017, the operators are hoping to produce the same number of collisions as in 2016, but over a shorter period, since the LHC has started up a month later due to the extended year-end technical stop.
“Over the first two years of operation at a collision energy of 13 TeV, we built up an excellent understanding of how the LHC works, which will allow us to optimise its operation even further in the third year,” says Frédérick Bordry, Director for Accelerators and Technology at CERN. “Our goal is to increase the peak luminosity even further and to maintain the LHC’s excellent availability, which in itself would be a great achievement.”
An image of a proton–proton collision taken in the CMS detector on 23 May. (Image: CMS/CERN)
Particle physics relies on the statistical analysis of various phenomena, so the size of the samples is crucial. In other words, the greater the number of collisions that reveal a certain phenomenon, the more reliable the result is. The experiments intend to take advantage of the large quantity of data supplied by the LHC to continue their exploration of physics at the highest energy ever obtained by an accelerator.
“The LHC experiments are well prepared to double their statistics compared to what they obtained in 2016 at 13 TeV. Thanks to the new data, they will be able to reduce the uncertainties that surround their observations every time we enter unchartered territory,” says Eckhard Elsen, Director for Research and Computing.
The LHC physicists are working on two different broad areas: improving their knowledge of known phenomena and probing the unknown. The known phenomena constitute the Standard Model of Particles and Forces, a theory that encompasses all our current knowledge of elementary particles. The Higgs boson, discovered in 2012, plays a key role in the Standard Model. It is also a scalar particle, fundamentally different to the other elementary particles. In 2017, ATLAS and CMS will continue to work on determining the characteristics of this particle. These two large general-purpose experiments will observe its decay modes and how it interacts with other particles. Their measurements may provide indications of possible new physics beyond the Standard Model. The experiments will also carry out precise measurements of other processes of the Standard Model, in particular those involving the top quark, the elementary particle with the greatest mass.
Physicists hope to be able to identify disparities between their measurements and the Standard Model. This is one of the ways in which the unknown can be probed. Although it describes a lot of the phenomena of the infinitely small precisely, the Standard Model leaves many questions unanswered. For example, it describes only 5% of the universe; the rest is formed of dark matter and dark energy, the nature of which are as yet unknown.
Every discrepancy with regard to the theory could direct physicists towards a larger theoretical framework of new physics that might resolve the enigmas we face.
One of the early collision events with stable beams recorded by ATLAS on 23 May 2017, with a reconstructed muon candidate. The upper panes show transverse views of the detector and the muon spectrometer, while the lower panes show ATLAS in longitudinal cross-section and a view of the energy deposits in the cells of the ATLAS calorimeters. (Image: ATLAS/CERN)
ATLAS, CMS and LHCb measure processes precisely to detect anomalies. ATLAS and CMS are also looking for new particles, such as those predicted by the theory of supersymmetry, which could be the components of dark matter.
LHCb is also interested in the imbalance between matter and antimatter. Both of these would have been created in equal quantities at the time of the Big Bang, but antimatter is now practically absent from the universe. LHCb is tracking the phenomenon known as “charge-parity violation” which is thought to be at least partly responsible for this imbalance.
No lead ion collisions, which are the ALICE experiment’s specialist subject, are planned at the LHC this year. ALICE will continue its analysis of the 2016 data and will record proton-proton collisions, which will also allow it to study the strong force. On the basis of the proton-proton collisions from 2016, ALICE recently announced that it had observed a state of matter resembling quark-gluon plasma. Quark-gluon plasma is the state of matter that existed a few millionths of a second after the Big Bang.
Finally, several days of physics running with de-squeezed beams are planned for the TOTEM and ATLAS/ALFA experiments.
To find out more about physics at the LHC, you can watch our Facebook Live event tomorrow Wednesday 24 May at 4 pm CEST.
One of the first proton-proton collisions seen by the ALICE experiment in 2017, on May 13, during the LHC beam commissioning phase. ALICE used these first collisions to fine-tune its equipment and get ready for the new physics season of LHC. (Image: CERN)
Last week, the detectors of the Large Hadron Collider (LHC) witnessed their first collisions of 2017. These test collisions were not for physics research, instead they were produced as part of the process of restarting the LHC. But have patience, data taking for physics will start in another few days.
Since particles began circulating in the large ring once more, the LHC’s operators have been testing and adjusting 24 hours a day to turn the LHC into a veritable collision factory. Their work involves forming trains of bunches, building them up over the next few weeks to several hundred and then several thousand bunches per beam.
To establish this production line of particles, all of the accelerator’s systems must be perfectly adjusted. The LHC is an extremely complex machine comprising thousands of subsystems and it takes weeks to adjust them all.
This image shows a beam splash, as observed by the ATLAS experiment on 29 April, the day of the LHC restart. The beam splashes are generated by aiming beams at the collimators near to the experiments, in this case 140 metres from the ATLAS interaction point. Once the LHC is back in operation, the experiments use the beam splashes to synchronise their sub-detectors with the accelerator’s clock. (Image: CERN)
The first particles circulated on 29 April 2017 and, soon after, the operators started work on their long list of adjustments. They tested the radiofrequency system, which accelerates the particles. They brought the beam energy up to its operating value of 6.5 TeV. They tested the beam dump system, which ejects the particles into a block of graphite if required. They tested and aligned all the collimators – jaw-like devices that close around the beam to absorb stray particles. They carried out proton bunch ramp and squeeze cycles. Finally, they performed fine adjustments of the hundreds of corrector magnets, adjusting the trajectory of the beam to a precision of one micron at the collision points.
Last Wednesday, they started to collide the beams to be able to adjust the interaction points at the heart of the experiments. This step is carried out with so-called “pilot” beams, containing fewer than ten bunches and fewer protons than during the physics runs. These first collisions also allow the experiments to adjust their detectors.
In the coming days, the operators will continue to adjust and align the equipment. Once all of these steps are complete, they will be able to announce “stable beams”, the long-awaited signal for the start of the new data-taking season for the experiments.
A beam splash, as observed by the CMS experiment on 29 April. In contrast to proton-proton collisions where the particles come from the center of the detector, in splash events particles traverse the detector horizontally, from one side to the other. (Image: CERN)
Final tests were performed in the LHC at the end of April, ready for the restart this weekend (Image: Maximilien Brice/ CERN)
Today, the LHC once again began circulating beams of protons, for the first time this year. This follows a 17-week-long extended technical stop.
Over the past month, after the completion of the maintenance work that began in December 2016, each of the machines in the accelerator chain have, in turn, been switched on and checked until this weekend when the LHC, the final machine in the chain, could be restarted by the Operations team.
“It’s like an orchestra, everything has to be timed and working very nicely together. Once each of the parts is working properly, that’s when the beam goes in, in phases from one machine to the next all the way up to the LHC,” explains Rende Steerenberg, who leads the operations group responsible for the whole accelerator complex, including the LHC.
Each year, the machines shut down over the winter break to enable technicians and engineers to perform essential repairs and upgrades, but this year the stop was scheduled to run longer, allowing more complex work to take place. This year included the replacement of a superconducting magnet in the LHC, the installation of a new beam dump in the Super Proton Synchrotron and a massive cable removal campaign.
Among other things, these upgrades will allow the collider to reach a higher integrated luminosity – the higher the luminosity, the more data the experiments can gather to allow them to observe rare processes.
“Our aim for 2017 is to reach an integrated luminosity of 45 fb-1 [they reached 40 fb-1 last year] and preferably go beyond. The big challenge is that, while you can increase luminosity in different ways – you can put more bunches in the machine, you can increase the intensity per bunch and you can also increase the density of the beam – the main factor is actually the amount of time you stay in stable beams,” explains Steerenberg.
In 2016, the machine was able to run with stable beams – beams from which the researchers can collect data – for around 49 per cent of the time, compared to just 35 per cent the previous year. The challenge the team faces this year is to maintain this or (preferably) increase it further.
The team will also be using the 2017 run to test new optics settings – which provide the potential for even higher luminosity and more collisions.
“We’re changing how we squeeze the beam to its small size in the experiments, initially to the same value as last year, but with the possibility to go to even smaller sizes later, which means we can push the limits of the machine further. With the new SPS beam dump and the improvements to the LHC injector kickers, we can inject more particles per bunch and more bunches, hence more collisions,” he concludes.
For the first few weeks only, a few bunches of particles will be circulating in the LHC to debug and validate the machine. Bunches will gradually increase over the coming weeks until there are enough particles in the machine to begin collisions and to start collecting physics data.
Learn more about the restart:
Rende Steerenberg is in charge of the Operations teams, who today are back in the CERN Control Centre as the LHC is turned on with beam for the first time in 2017 (Image: Sophia Bennett/CERN)
Rende Steerenberg is affectionately known around CERN as the man who pushes the button to restart the LHC, but he is emphatic that this isn’t the case.
“It’s not just my job, and it’s not just one button,” he grins, after hearing the accolade. Rende’s real job title is leader of the Operations group – a team that make sure the LHC and the accelerator chain, together with the technical infrastructure, are running smoothly.
Today, the Large Hadron Collider restarted after almost five months of maintenance and upgrades. And the reason Rende (rightly) claims it’s not just one button, is because starting up the LHC requires a slow process of switching on each individual part of the accelerator chain in turn, until it gets to the final, biggest machine.
“I did a lot of Alpinism before the kids were born. It’s quite dangerous so I stopped once they were around, but we still go hiking with them in the mountains a lot,” says Rende Steerenberg, who runs the Operations group at CERN. “Like in my private life, in our daily work we have to take responsibility for our actions.” (Image: Sophia Bennett/CERN)
“To get things done here you have to run the accelerator but you can’t work alone. There are many people working day and night to run the accelerators. And you have to be in contact with the Proton Synchroton operator, who has to be in contact with the SPS or the Booster operator, and if something is not right they all have to work together.”
Giving credit where it’s due is crucial to Rende’s role as a manager, and something he enjoys so much he believes that even if he hadn’t ended up in a science role, he’d have been in management somehow, working in a team.
“As the Group Leader I have to manage the group and the resources and make sure everything can work, that people have the right tools, hiring, etc. It all takes a lot of my time. But I’m still very closely involved in the running of the machines, discussions about increasing the performance of the injectors, producing brighter beams, increasing the LHC’s integrated luminosity, it’s all still part of my job, which is fortunate as that’s a very interesting part of it too.” – Rende Steerenberg speaking about managing the teams who meet in the CCC shown above (Image: Sophia Bennett/CERN)
The operations teams work in shifts at CERN’s Control Centre (the CCC), where each corner of the room is an island of computer screens devoted to a specific machine. It’s in the CCC that the social, team-building nature of Rende’s group becomes clear.
“My favourite bit of my working day is coming into the CCC in the morning. I scan through the logbooks over breakfast but then I come in and listen to the people who run the machines about how the night went. What were the issues, and the successes?” shares Rende.
“It’s a hugely well-functioning dynamic, people bring pasta to night shifts!” – Rende Steerenberg
For him, it’s made even more enjoyable when it’s a point of shift turnover, when different people are passing through the CCC. Working in shifts, seeing people bring in shared pasta dishes to sustain them through long nights, is what Rende, among other things, thinks gives the team such a well-functioning dynamic.
“It’s a different life in a control room to working in an office. If we had an argument, you might hide in your office, me in mine and one of us would call and say, ‘Can we discuss it again over coffee?’ But that might take days. In the control room that can’t happen, you have to meet up the next night when no-one else is around, you have to sit next to each other and you have to work out issues in the same island. It’s a special atmosphere, more family like.”
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Preparations being made in the underground experimental cavern of CMS prior to the installation of the second-generation Pixel Tracker of CMS
Scientists at CERN have now completed “open-heart surgery” on one of the detectors at the Large Hadron Collider (LHC). In a complex operation that ran from 27 February to 9 March, the giant Compact Muon Solenoid (CMS) detector received a new “heart” – it’s Pixel Tracker.
Detectors at the LHC, such as CMS, record the signatures of particles produced when beams of protons (or, occasionally, lead nuclei) are smashed together. The detectors are built around the LHC’s beam pipe, within which the collisions take place. As the particles fly through the detectors, they traverse several layers of equipment that are tasked with making specific measurements about their properties. But, when these collisions occur, it isn’t a single proton hitting another proton: several dozen simultaneous collisions take place within CMS. This phenomenon is known as “pile-up” and can be thought of as exposing a film camera to multiple images and recording all the multiple exposures in a single photograph.
The tracking system determines the trajectories of charged particles flying through it, and identifies the charge and momenta of the particles, helping to determine the origins of the various particles seen by CMS. Physicists can thus separate the overlapping collisions into individual interactions.
The CMS tracking system is made of silicon sensors and has two components that perform a complementary roles: the inner of the two is called the Pixel Tracker and the outer one is the Strip Tracker. The Pixel Tracker sees the greatest onslaught of particles flying through CMS and, unavoidably, it will lose its ability to measure the particles’ properties accurately. In addition, the LHC continues to improve its performance and is expected to provide CMS with an even greater number of simultaneous interactions: even more exposures on each photograph. It had therefore been planned around five years ago to replace the original Pixel Tracker of CMS, removed earlier this year, with an entirely new one.
The new Pixel Tracker has four layers instead of the previous three in the central region (called BPIX for Barrel PIXel) and has three disks instead of the previous two capping each end (called FPIX for Forward PIXel). These additional layers raise the number of silicon pixels in CMS from 66 million to 124 million, increasing the “resolution” of the “photographs” CMS takes, so to speak.
The FPIX disks were manufactured by 19 institutes in the US. They can be seen here at the CMS Tracker Integration Facility at Meyrin, Switzerland before being taken to the CMS experimental site outside Cessy, France for installation. The Pixel Tracker’s various components were stored and tested carefully on the surface in a clean room prior to installation. (Image: Maximilien Brice/CERN)
To be installed within CMS, the various components of the Pixel Tracker had to be lowered by crane down the 100-metre-deep shaft into the underground experimental cavern of CMS. They were then raised by a second crane onto the installation platform for insertion. This image shows the first half of the BPIX located inside its “cassette” being placed on this platform before being inserted into the CMS detector. The BPIX, manufactured by 23 institutes from eight European countries, is only the size of a shoebox, but has a large number of electronics and cooling components that go with it. (Image: Maximilien Brice/CERN)
Once lowered onto the installation platform, the protective coverings of the device was removed and it was slowly and carefully slid into place around the LHC beam pipe. Here, the second half of BPIX is being prepared for insertion. (Image: Maximilien Brice/CERN)
The LHC beam pipe can be seen prominently in this picture with the two halves of BPIX fitting snugly around it. The particle beams of the LHC fly within this beam pipe before colliding with each other inside CMS. 6. (Image: Maximilien Brice/CERN)
Surgery in action! Appropriate protection during installation of the FPIX prevents contamination of the device. (Image: Maximilien Brice/CERN)
The many wires and electronics connected to the Pixel Tracker’s active components had to be thoroughly checked during the installation procedure and had to be moved into place delicately. (Image: Maximilien Brice/CERN)
The installation of the final FPIX component brings the long operation of replacing the CMS Pixel Tracker to a successful end. CMS will soon be moved into its data-taking configuration to prepare for the first proton-proton collisions of 2017, expected in early June. (Image: Maximilien Brice/CERN)
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)