First collisions of lead ions in the LHC at record energy

On Friday 18 November, a test using lead ion collisions was carried out in the LHC and provided the experiments with an opportunity to validate the new detectors and new data processing systems ahead of the next lead-lead physics race year.

Event visualizations of Run3’s first Pb-Pb collision on November 18, 2022 (Image: CERN)

After the successful start of Run 3 in July this year, which saw proton-proton collisions at a record energy of 13.6 TeV, it was the turn of lead nuclei to circulate again in the Large Hadron Collider (LHC) lo last Friday after a gap of four years. The lead nuclei comprise 208 nucleons (protons and neutrons) and are used at the LHC to study quark-gluon plasma (QGP), a state of matter in which the building blocks, quarks and gluons, are not confined within of nucleons but can move and interact over a much larger volume.

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Visualization of the event of a lead-argon collision in the LHCb (Image: CERN)
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Event visualization 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 from the LHC. (Image: CERN)

In the test carried out last Friday, the lead nuclei were accelerated and collided to a record energy of 5.36 TeV per nucleon-nucleon collision1. This is an important milestone in preparation for the lead-to-lead collision physics runs planned for 2023 and the subsequent years of Run 3 and Run 4.

CERN’s ion injector complex underwent a series of upgrades in preparation for doubling the total lead ion beam intensity for the high-luminosity LHC. Achieving this goal requires a technique called “momentum slip-stacking” to be used in the Super Proton Synchrotron (SPS), in which two batches of four groups of lead ions separated by 100 nanoseconds “slip by” to produce a single batch of 8 lead groups separated by 50 nanoseconds. This will allow the total number of clusters injected into the LHC to increase from 648 in Run 2 to 1248 in Run 3 and later. After all upgrades have been completed, the LHC will deliver ten times as many heavy ion collisions as in past Runs.

The test was also a crucial milestone for ALICE, the LHC experiment specializing in the study of lead-ion collisions. The ALICE apparatus was upgraded during the recent LHC shutdown and now features several entirely new or greatly improved detectors, as well as new data processing hardware and software. The new detectors provide greater spatial resolution in the reconstruction of the trajectories and properties of the particles produced in the collisions. In addition, the upgraded apparatus and upgraded processing chain can record the full collision information at two orders of magnitude faster speed.

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

Other experiments used the test run to commission their upgraded and newly installed subsystems in the new heavy ion environment of higher energy and 50 ns group spacing. ATLAS tested updates to its selection (trigger) software, which is designed to improve heavy ion physics data during Run 3. Specifically, the physicists tested a new particle track trigger designed to locate a range broader than “ultra peripheral collisions”. CMS has upgraded several components of its readout, data acquisition, triggering and reconstruction chains to be able to take full advantage of high energy lead-lead collisions. Lead-lead fills provided by the LHC have enabled CMS to commission the entire system with beam and pinpoint areas that could be further optimized for the 2023 heavy ion rushes. LHCb has begun putting its brand new detector into operation under the challenging collision conditions lead-lead characterized by a very large multiplicity of particles.In addition to the lead-lead collisions, the LHCb collected the collisions the lead-argons in fixed target mode using the new SMOG2 system, which is unique to the experiment and is designed to inject noble gases into the LHCb collision area.

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

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

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