Restarting the LHC: a new era of physics data
Precision technology to unravel the mysteries of the Universe – also made in Switzerland
After three years of a scheduled break, the Large Hadron Collider LHC at CERN is back at full throttle, accelerating particle beams at record energies and since 5 July 2022 producing first collisions for physics analyses. Institutes from all over Switzerland have contributed to the upgrading of the enormous particle physics complex and scientists are keen to their hands on the new data it will produce in its new run.
“Collisions! We have collisions!” Scientists and engineers in control rooms around CERN and in front of their computer screens around the world were over the moon when the LHC declared “stable beams for collisions” for the first time in three years this week. The 27-kilometre particle accelerator had been switched off for maintenance and upgrades, and the collaborations running the four detectors along its ring used the time to get their gigantic particle cameras that capture every collision up to speed as well.
First low-intensity particle bunches were injected into the LHC accelerator around Easter, and after rounds of thorough tests and careful ramping up of the beam energy and the particle count, the machine operators felt confident to deliver the first collisions to the high-tech detectors on 5 July. This is the official start of Run 3 of the LHC – a planned four-year period that, scientists hope, will reveal more details of the Higgs boson and the inner workings of the Standard Model, and will possibly give hints to unexpected and rare events that could teach us more about the fundamental rules that govern our universe as well as its history and possible fate.
So what were the changes made to machine and detectors? Here’s an overview…
When a high-tech device runs for months at a time and around the clock, many materials wear out. It’s this expected wear and tear that was tackled during the lockdown-prolonged shutdown, alongside some additions to the accelerator complex to make it more efficient and deliver more collisions at slightly higher energies. The pre-accelerators to the LHC received new magnets, upgrades to their acceleration systems and new beam dumps. A new linear accelerator was connected up at the start of the acceleration complex and vast amounts of general maintenance work was completed, so the accelerators are now ready for their next run time.
The LHC itself is now more powerful and more reliable because the electrical insulation of diodes connecting the 1200 superconducting magnets has been improved, some magnets have been replaced, better cryogenic power devices and new tools to monitor and improve the beam quality have been installed. With all these changes in place, the experiments can expect to collect data from more collisions during this physics run than in the two previous physics runs combined.
LHCb: ready for the data flood after a major overhaul
Among the four major experiments at LHC, the LHCb detector is the one that was refurbished the most. LHCb looks for hints of tiny differences between particles and antiparticles, especially those containing a b- or c-quark, to answer the question why the world consists only of matter even though matter and antimatter must have been produced in equal amounts at the big bang. Slight imbalances in the behaviour or properties of matter and antimatter particles could account for this. In order to learn all about them, the detector has a very special setup.
LHCb isn’t built like an onion like the other detectors, where concentric layers are arranged around the collision point. Instead, it is arranged in vertical slices. The first subdetector sits very close to the collision point, the others follow one after another over a length of 20 metres. This arrangement catches mainly “forward“ particles – those particles produced in collisions and emitted at small angles with respect to the beam, in the direction of the detector.
Sci-Fi –Science fiction? Yes, and scintillating fibres
With more collisions on the horizon, it was time for the LHCb collaboration adopt a complete change in data-acquisition strategy and to replace some of their key components. One of these replacements is the Scintillating Fibre (SciFi) Tracker, a complex detector system consisting of twelve detection planes of a total of 11 000 kilometres of scintillating fibres and some 340 m2 of active detection surface. It sits around eight metres away from the interaction point behind the magnet and detects signals of charged particles whose trajectories have been bent by the magnet. The previous tracker with silicon strip detectors in the inner region and gas detectors in the outer region was not designed to cope with the expected increase in particle rate. “That was one of the challenges”, recounts Oliver Schneider from EPFL in Lausanne. “Replacing the gas detectors with silicon strip detectors for such a large area was too expensive, so we needed a different technology, able to cope with large particle rates and a high-radiation environment, at affordable cost. It didn’t exist, so we had to invent and build our own.”
Their solution to the problem: take a standard plastic optical fibre, add a scintillator, wind them on a big wheel in dense layers to a width of 15 centimetres and a length of 2.5 metres, connect all the layers with glue and repeat until the fibres form a mat that is 1.5 millimetres thick. The result is a scintillating fibre mat that is then assembled into modules and connected up to silicon photomultipliers (SiPMs). The SciFi tracker needed a total of 1500 mats, 500 of these were produced in Lausanne. “It’s a complex manufacturing process for which we had to build up production sites in three labs in Europe. It took more than two years to produce them”, says Fred Blanc, also from EPFL.
The result is a detector that is its own data transmitter. It consists only of fibres, no “dead” material or electronics that could disturb or absorb interesting tracks from the collisions. “That makes it very elegant. It’s very uniform and has little mass,” says Olivier Schneider. The SiPM sensors were also pushed to their limit because they have never been used in a high-radiation environment before. “We found that if we cooled them down to minus 40 degrees they would survive their designated lifetime after proper training”, explains EPFL’s Guido Haefeli. “All the SiPMs for the SciFi come from EPFL, so our students learned a lot of hands-on detector technology in the last years. Now we are all curious to see if it works and ages as we expect.”
Other parts of LHCb have also been kitted out with new silicon technology, for example the smaller tracker before the magnet, in which the University of Zurich is involved. Even closer to the collision point, the “Vertex Locator” or VELO has been rebuilt with silicon pixel detectors. This device sits closest to the proton beams – it’s only 5 millimetres away from it! It needs to be this close because it captures extremely short-lived particles containing b- or c-quarks and the points where they decay into other particles very precisely.
In fact, the LHCb detector went through a major upgrade driven by the necessity of increasing significantly the data-taking efficiency. For this a radical change in data acquisition strategy is adopted by abandoning the first level of the trigger system, a kind of gatekeeper that only lets potentially interesting collisions pass into LHCb’s data acquisition system, meaning a lot of data was lost. Now, this trigger is gone and LHCb can read out all channels for each collision event, meaning every 25 nanoseconds. To do this, most of the detectors readout electronics were changed. With all these changes in place, the physicists can’t wait to get their hands on the new Run-3 collision data. “With this increased efficiency we can record a much larger sample of particles containing b- and c-quarks and improve the precision of our measurements,” says Blanc. “The whole collaboration is excited to get going.”
CMS: more power for the detector’s heart
Lea Caminada is a particle physicist through and through, but during shutdowns, she becomes a heart surgeon – for the heart of the CMS detector. CMS needed a new, innermost layer for its innermost subdetector, the pixel detector, and Caminada, who works at PSI and the University of Zurich, is the project leader for this major undertaking. The pixel detector’s main goal is to make precise measurements of the tracks of charged particles flying out from the collisions and the places from where they originate. With this information scientists can get a full picture of all the interactions during a collision. The pixel detector will be able to do this job even more efficiently after its overhaul during the shutdown.
With its position directly surrounding the beampipe, the pixel detector is exposed to a lot of damaging radiation and needs more tender loving care than many other components. It was completely replaced during a previous shutdown and in this shutdown has been equipped with a new innermost layer consisting of silicon detector modules built at PSI. This layer has about a hundred modules, each consisting of 66 000 pixels, contributing their part to the total of 120 million pixels in the pixel detector. The complete subdetector was removed from CMS and stored until the layer was ready to be put in in 2021. “It took more than two years including some pandemic-related delays,” says Caminada. “We improved the readout electronics, tested and calibrated all modules at the operating temperature of minus 20 degrees as well as at room temperature, mounted them and cabled it all up. Now we have a detector with a much better capacity to cope with the high rates of collisions coming in from the LHC.”
The new layer was installed in summer 2021. Since then the team tested the functionality of the detector, calibrated all channels and operated the detector with cosmic rays. And it’s looking very good. Caminada says, “We are running at very nearly 100% efficiency and are ready for data – that should be amazing for the new run!”
ATLAS: in the race with new “small wheels”
The main change during the recent long shutdown to the gigantic ATLAS detector were two new “small” wheels – which measure 10 metres in diameter and weigh 100 tonnes each. They will improve ATLAS’ alert system for interesting collisions and will be able to cope with the higher muon rates expected from the high-luminosity LHC, to come after the next shutdown in about four years.
While they weren’t involved in the construction of these small wheels, Swiss institutes (in this case the University of Geneva) contributed to the alarm system, or trigger, mentioned above, and the University of Bern in working on a new silicon tracker that is also already a preparatory measure for the high-luminosity LHC. “We need to deal with a much higher data rate in the future, so a highly efficient optical readout for the silicon technology is absolutely crucial,” says Hans Peter Beck from University of Bern. Their advantage: In Bern they can run many tests in their in-house cyclotron, for example the radiation hardness of cables to be inserted into the tracker.
Beck and his colleagues from around the world are still busy analysing the data from the last run – some 500 analyses are still in the works, he reckons. Nevertheless he is very much looking forward to what the LHC has in store with run 3. The slight increase in energy can be decisive, he says, and by the end of 2025 the LHC will have produced twice the amount of data than in all of its previews runs put together. “News about the Higgs or hints of supersymmetry, dark matter or leptoquarks could all be lurking round the energy and intensity corner. There’s a lot of food for our studies – and for thought!”
Author: Barbara Warmbein
Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Ben Kilminster
Department of Physics