First lead ion collisions in the LHC at record energy

On Friday 18 November, a test using lead ion collisions was carried out in the LHC and allowed the experiments to validate the new detectors and the new data processing systems before the next lead-lead physics campaign.

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Run3’s first Pb-Pb collision event displays taken on November 18, 2022 (Picture: CERN)

After the successful start of Run 3 in July this year, which featured proton-proton collisions at the record energy of 13.6 TeV, it was the turn of lead nuclei to circulate again in the Large Hadron Collider (LHC) last Friday after a four-year hiatus. Lead nuclei consist of 208 nucleons (protons and neutrons) and are used at the LHC to study quark-gluon plasma (QGP), a state of matter in which the elementary constituents, quarks and gluons, are not confined within nucleons but can move and interact on a much larger volume.,Experiments and Tracks
Event display of a lead-argon collision in LHCb (Picture: CERN)
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Display of a heavy ion collision event recorded in ATLAS on November 18, 2022, when stable beams of colliding lead ions at a center-of-mass energy per nucleon pair of 5.36 TeV were delivered to ATLAS by the LHC. (Picture: CERN)

In the test carried out last Friday, lead nuclei were accelerated and collided at a record energy of 5.36 TeV per nucleon-nucleon collision1. This is an important step in the preparation of the physics tests with lead-lead collisions planned for 2023 and the following years of tests 3 and 4.

CERN’s ion injector complex has undergone a series of upgrades to double the total intensity of lead ion beams for the High-Luminosity LHC. Achieving this goal requires a technique called “momentum slip stacking” for use in the Super Proton Synchrotron (SPS), where two batches of four bunches of lead ions separated by 100 nanoseconds “slip” to produce a single batch of 8 lead packets. separated by 50 nanoseconds. This will allow the total number of bunches injected into the LHC to increase from 648 in Run 2 to 1248 in Run 3 and beyond. When all upgrades are complete, the LHC will deliver ten times more heavy ion collisions compared to previous runs.

The test was also a crucial step for ALICE, the LHC experiment specializing in the study of lead-ion collisions. The ALICE device was upgraded during the recent LHC shutdown and now includes several completely new or greatly improved detectors, as well as new hardware and software for data processing. The new detectors offer higher spatial resolution in reconstructing the trajectories and properties of particles produced during collisions. Additionally, the upgraded device and upgraded processing chain can record full collision information at a rate two orders of magnitude higher.

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Events seen in the CMS detector from Pb-Pb collisions (Picture: CERN)

Other experiments used the test to commission their upgraded and newly installed subsystems in the new heavy ion environment of higher energy and 50 ns bunch spacing. ATLAS tested upgrades to its picker (trigger) software, which is designed to improve heavy ion physics data taking during Run 3. In particular, physicists tested a new trigger tracking particles designed to detect a wider range of “outermost collisions”. ”. CMS has upgraded several components of its readout, data acquisition, trigger and reconstruction chains to take full advantage of high-energy lead-lead collisions. The lead-lead fillings delivered by the LHC enabled CMS to commission the entire beam system and identify areas that could be further optimized for the heavy ion cycles of 2023. LHCb began commissioning service its brand new detector in the difficult conditions of lead-lead collisions characterized by a very high multiplicity of particles. In addition to lead-lead collisions, LHCb collected lead-argon collisions in fixed target mode using the new SMOG2 system, unique to the experiment and designed to inject noble gases into the LHCb collision zone.

Although very short, the lead-lead 2022 program can be considered a success for the LHC accelerator, the experiments and the heavy ion injector complex at CERN. The LHC’s four large detectors saw and recorded lead-lead collisions at record energy for the first time. Researchers are now looking forward to the heavy ion physics campaign in 2023 and beyond.

1 In lead-lead collisions, each of the 208 nucleons of one of the lead nuclei can interact with one or more nucleons of the other lead nucleus.

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