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First lead-ion collisions in the LHC at record energy

Tue, 22/11/2022 - 12:29
First lead-ion collisions in the LHC at record energy Event displays of the first Pb-Pb collision of Run3 taken on 18 November 2022 (Image: 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 in the Large Hadron Collider (LHC) again last Friday after a gap of four years. 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 elementary constituents, quarks and gluons, are not confined within nucleons but can move and interact over a much larger volume.

Event display of a lead-argon collision in LHCb (Image: CERN) Event display of a heavy ion collision event recorded in ATLAS on 18 Nov 2022, when stable beams of lead ions colliding at a center-of-mass energy per nucleon pair of 5.36 TeV were delivered to ATLAS by the LHC. (Image: 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 milestone in preparation for the physics runs with lead-lead collisions that are planned for 2023 and the following years of Run 3 and Run 4.

The CERN ion injector complex has undergone a series of upgrades in preparation for a doubling of 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 bunches separated by 100 nanoseconds “slip” to produce a single batch of 8 lead bunches 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 onwards. After all the upgrades have been completed the LHC will provide a ten-fold higher number of heavy ion collisions with respect to the past Runs.

The test was also a crucial milestone for ALICE, the LHC experiment that specialises in the study of lead-ion collisions. The ALICE apparatus was upgraded during the recent shutdown of the LHC and now features several completely new or greatly improved detectors, as well as new hardware and software for data processing. The new detectors provide a higher 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 a rate two orders of magnitude higher.

Events as 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 50ns bunch spacing. ATLAS tested upgrades to its selection (trigger) software, which is designed to enhance heavy-ion-physics data taking in Run 3. In particular, physicists tested a new particle-track trigger designed to spot a wider range of “ultra-peripheral collisions”. CMS upgraded several components of its readout, data acquisition, trigger and reconstruction chains to be able to take full advantage of the high-energy lead-lead collisions. The lead-lead fills delivered by the LHC allowed CMS to commission the entire system with beam and spot the areas that could be further optimized for the 2023 heavy-ion runs. LHCb started commissioning its brand-new detector in the challenging conditions of lead-lead collisions characterised by a very large particle multiplicity. In addition to lead-lead collisions, LHCb collected lead-argon collisions 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.

Even if very short, the 2022 lead-lead programme can be considered a success for the LHC accelerator, the experiments and CERN's heavy-ion injector complex. The four big LHC detectors saw and recorded lead-lead collisions at a new record energy for the first time. Researchers are now looking forward to the heavy-ion physics campaign in 2023 and the following years.

 

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

ptraczyk Tue, 11/22/2022 - 11:29 Byline ALICE collaboration Publication Date Wed, 11/23/2022 - 11:22

LHCb’s new VELO springs into action

Wed, 26/10/2022 - 11:56
LHCb’s new VELO springs into action

On Friday, 21 October, at 10.15 p.m. CEST, the Large Hadron Collider beauty (LHCb) experiment passed an important milestone for Run 3. Its state-of-the-art vertex locator, or VELO upgrade, aligned more closely with the LHC beam than ever before. This process, known in LHCb jargon as “VELO closing”, allows the experiment to reconstruct the trajectories of the particle collisions at LHCb with extreme precision.

LHCb analyses particles thrown forward from the collision point of the two LHC beams. In particular, the LHCb collaboration searches for a type of particle called a B meson, which is characterised by containing a “beauty” quark. B mesons are important for particle physics research because their interactions may hold the clues to the limitations of the Standard Model, which currently dictates all of particle physics. Earlier this year, using data from the previous version of the detector, LHCb announced the discovery of new types of matter–antimatter asymmetry and new exotic particles. Both of these topics, and many more, will be probed further with the new detector.

To be able to understand these particle interactions fully, physicists need more data. The VELO’s job is to completely reconstruct the trajectories of particle collisions, pick out the important interactions involving B mesons, and analyse them.

“It has to be incredibly close to the beams in order to get the maximum accuracy,” says Paula Collins, LHCb experimental physicist. The VELO achieves this by gradually moving pairs of plates closer to the beam, so close that they even enter the vacuum of the LHC beam pipe. The subdetector is composed of millions of pixels, which act like a camera, taking pictures of the interaction at a rate of 40 million times per second.

The plates start at a width of about 3 cm apart, and carefully move to centre around the beam. “When we’re closed, the aperture where the LHC beams pass is just 3.5 mm,” continues Collins. “This is something like the diameter of a pencil, and the 400 mega-joule LHC beams have to pass through this very narrow space.”

This impressive feat could not have been achieved without a huge team effort to design, install and operate the VELO. The LHCb detector underwent a complete overhaul in preparation for Run 3 of the LHC, which began on 5 July 2022. The VELO is only one of a number of brand-new subdetectors that have increased LHCb’s precision and data-taking capacity. Other new subdetectors include the new upstream tracker (UT) and the scintillating fibre tracker (SciFi), which analyse the beam either side of the central LHCb magnet.

Since the LHC began, physicists have discovered the existence of 68 new hadrons, 60 of which were discovered by the LHCb experiment. The unprecedented accuracy achieved by the new VELO marks an exciting new era for the experiment, with hope for plenty more discoveries to come.

You can find out more about this milestone in the video below:

Alignment of the LHCb experiment's VELO subdetector (Video: CERN)

 

ndinmore Wed, 10/26/2022 - 10:56 Byline Naomi Dinmore Publication Date Wed, 10/26/2022 - 10:42

LHCb’s new VELO springs into action

Wed, 26/10/2022 - 11:56
LHCb’s new VELO springs into action

On Friday, 21 October, at 10.15 p.m. CEST, the Large Hadron Collider beauty (LHCb) Upgrade I experiment passed an important milestone for Run 3. Its state-of-the-art vertex locator, or VELO upgrade, aligned more closely with the LHC beam than ever before. This process, known in LHCb jargon as “VELO closing”, allows the experiment to reconstruct the trajectories of the particle collisions at LHCb with extreme precision.

LHCb analyses particles thrown forward from the collision point of the two LHC beams. In particular, the LHCb team searches for a type of particle called a B meson, which is characterised by containing a “beauty” quark. B mesons are important for particle physics research because their interactions may hold the clues to the limitations of the Standard Model, which currently dictates all of particle physics. Earlier this year, using data from the previous version of the detector, LHCb announced the discovery of new types of matter–antimatter asymmetry and new exotic particles. Both of these topics, and many more, will be probed further with the new detector.

To be able to understand these particle interactions fully, physicists need more data. The VELO’s job is to completely reconstruct the trajectories of particle collisions, pick out the important interactions involving B mesons, and analyse them.

“It has to be incredibly close to the beams in order to get the maximum accuracy,” says Paula Collins, LHCb experimental physicist. The VELO achieves this by gradually moving pairs of plates closer to the beam, so close that they even enter the vacuum of the LHC beam pipe. The subdetector is composed of millions of pixels, which act like a camera, taking pictures of the interaction at a rate of 14 million times per second.

The plates start at a width of about 3 cm apart, and carefully move to centre around the beam. “When we’re closed, the aperture where the LHC beams pass is just 3.5 mm,” continues Collins. “This is something like the diameter of a pencil, and the 400 mega-joule LHC beams have to pass through this very narrow space.”

This impressive feat could not have been achieved without a huge team effort to design, install and operate the VELO. The LHCb detector underwent a complete overhaul in preparation for Run 3 of the LHC, which began on 5 July 2022. The VELO is only one of a number of brand-new subdetectors that have increased LHCb’s precision and data-taking capacity. Other new subdetectors include the new upstream tracker (UT) and the scintillating fibre tracker (SciFi), which analyse the beam either side of the central LHCb magnet.

Since the LHC began, physicists have discovered the existence of 68 new hadrons, 60 of which were discovered by the LHCb experiment. The unprecedented accuracy achieved by the new VELO marks an exciting new era for the experiment, with hope for plenty more discoveries to come.

You can find out more about this milestone in the video below:

Alignment of the LHCb experiment's VELO subdetector (Video: CERN)

 

ndinmore Wed, 10/26/2022 - 10:56 Byline Naomi Dinmore Publication Date Wed, 10/26/2022 - 10:42