HomeSciencePhysicsFirst lead-ion collisions in the Large Hadron Collider at record energy

First lead-ion collisions in the Large Hadron Collider at record energy

Event views of Run3’s first Pb-Pb collision taken on November 18, 2022. Credit: CERN

On Friday, November 18, a lead ion collision test was conducted at the Large Hadron Collider (LHC) and provided an opportunity for the experiments to validate the new detectors and new data processing systems ahead of next year’s lead-lead physics run.

After the successful start of Run 3 in July of this year, in which proton-proton collisions at a record energy of 13.6 TeV, it was the turn of lead nuclei to recirculate in the LHC last Friday after a four-year hiatus. Lead nuclei contain 208 nucleons (protons and neutrons) and are used in the LHC to study quark gluon plasma (QGP), a state of matter in which the elementary constituents, quarks and gluons, are not confined to nucleons, but can move and interact over a much larger volume.

In the test conducted last Friday, lead nuclei were accelerated and collided with a record energy of 5.36 TeV per nucleon-nucleon collision. This is an important milestone in preparation for the lead-lead collision physical runs planned for 2023 and the following years of Run 3 and Run 4. In lead-lead collisions, each of the 208 nucleons of one of the lead nuclei can interact with one or multiple nucleons from the other leading nucleus.

The CERN ion injector complex has undergone a series of upgrades in preparation for doubling the total intensity of the lead ion beams for the High-Luminosity LHC. Achieving this goal requires a technique called “momentum slip-stacking” to be used in the Super Proton Synchrotron (SPS), where two batches of four lead-ion beams separated by 100 nanoseconds “slip” to produce a single batch of 8 lead bundles. separated by 50 nanoseconds.

This allows the total number of bunches injected into the LHC to increase from 648 in run 2 to 1248 in run 3 and beyond. After all upgrades are completed, the LHC will provide a tenfold increase in the number of heavy ion collisions compared to the previous runs.

First lead ion collisions at the LHC with record energy

vent of a lead-argon collision in LHCb. Credit: CERN

The test was also a critical milestone for ALICE, the LHC experiment that specializes in the study of lead-ion collisions. The ALICE device was upgraded during the recent closure of the LHC and now features a number of completely new or much improved detectors, as well as new data processing hardware and software.

The new detectors provide higher spatial resolution in the reconstruction of the trajectories and properties of the particles produced in the collisions. In addition, the upgraded device and the upgraded processing chain can record the full collision information at a rate two orders of magnitude higher.

First lead ion collisions at the LHC with record energy

Event view of a heavy ion collision captured in ATLAS on November 18, 2022, when stable beams of lead ions colliding with a centroid energy per nucleon pair of 5.36 TeV were delivered from the LHC to ATLAS. Credit: CERN

Other experiments used the to test running to deploy their upgraded and newly installed subsystems in the new environment with higher energy heavy ions and 50ns beam spacing. ATLAS has been testing upgrades to its selection software (trigger), which is designed to improve heavy ion physics data capture in Run 3. In particular, physicists have tested a new particle track trigger designed to detect a wider range of “ultra-peripheral collisions”. “.

CMS has upgraded several components of its readout, data acquisition, trigger, and reconstruction chains to take full advantage of the high-energy lead-to-lead collisions.

First lead ion collisions at the LHC with record energy

Events as observed in the CMS detector of Pb-Pb collisions. Credit: CERN

The lead-lead fills provided by the LHC enabled CMS to commission the entire system with beam and spot the areas that could be further optimized for the heavy ion runs in 2023. LHCb began commissioning its brand new detector in the challenging conditions of lead-lead collisions characterized by very high particle multiplicity. In addition to lead-lead collisions, LHCb collected solid-target lead-argon collisions using the new SMOG2 system, which is unique to the experiment and designed to noble gases in the LHCb crash Surface.

Even though it is very short, the 2022 lead-to-lead program can be considered a success for the LHC accelerator, the experiments and the CERN heavy ion injector complex. The four large LHC detectors saw and recorded lead-lead collisions with a new record energy for the first time. Researchers now look forward to the heavy ion physics campaign in 2023 and beyond.

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