After a five-year pause, on the evening of 26 September, lead ions collided at the Large Hadron Collider (LHC) at an unprecedented high energy of 5.36 TeV per pair of nucleons (protons or neutrons) and a collision rate six times higher than before. The final lead-ion beam of this latest heavy-ion run was dumped early in the morning of 30 October, after a forced magnet ‘quench’, carried out to better understand the amount of deposited energy at which the LHC superconducting magnets lose their superconducting state. This improved understanding of the LHC machine will help to further increase the heavy-ion collision rate in the near future.
For this much-anticipated heavy-ion run, alongside improved beam parameters, the ALICE experiment – the LHC’s heavy-ion specialist – made use of its significantly upgraded detector with continuous readout electronics. This means that each and every collision can now be recorded and is thus available for physics analysis, whereas, in the past, only a fraction of collisions could be selected for recording. This continuous readout was achieved by revamping the experiment’s time projection chamber (TPC) detector and upgrading the readout electronics of all of the detectors. In addition, the new inner tracking system (ITS) detector, which is based on highly granular silicon pixel technology, provides sharp images of the collisions with its 10 m2 of active silicon area and nearly 13 billion pixels within the three-dimensional detector volume.
The resulting dramatic increase in the data rate was facilitated by the deployment of a new computing infrastructure for online data processing. This infrastructure includes a new data processing farm that sends the data produced by the experiment directly to CERN’s Data Centre, located about five kilometres from ALICE, through a dedicated high-speed optical-fibre connection that had to be established to cope with the increased data rate.
During the five-week run, ALICE recorded about 12 billion lead–lead collisions – 40 times more collisions than the total recorded by ALICE in the previous periods of heavy-ion data taking, from 2010 to 2018. The new data processing farm, consisting of 2800 graphics processing units (GPUs) and 50 000 central processing unit (CPU) cores, routinely digested collision data at a rate of up to 770 gigabytes per second. It then compressed the data to about 170 gigabytes per second before shipping it to the Data Centre for storage on disk and later, at a limited speed of 20 gigabytes per second, for storage on tape for long-term preservation.
The fresh data set – which amounts to 47.7 petabytes of disk space and is now being analysed – will advance physicists’ understanding of quark–gluon plasma, a state of matter in which quarks and gluons roam around freely for a very short time before forming the composite particles called hadrons that ALICE detects. The increased number of recorded collisions will allow the ALICE researchers to determine the temperature of the plasma using precise measurements of thermal radiation in the form of photons and pairs of electrons and positrons. It will also allow other properties of the nearly-perfect fluid to be measured with greater precision, especially using hadrons containing heavy charm and beauty quarks.The number of lead–lead collisions collected by ALICE in 2023, expressed in terms of the cumulative number of collisions (right vertical axis) and a related quantity called integrated luminosity (left vertical axis). (Image: CERN) abelchio Tue, 11/28/2023 - 10:48 Byline ALICE collaboration Publication Date Fri, 12/01/2023 - 18:00
For the last five weeks, the Large Hadron Collider (LHC) delivered lead-ion beams to the experiments, marking the first-ever heavy-ion run at the record energy of 5.36 TeV per nucleon pair and the first of the LHC Run 3.
By observing the particles created in lead-lead collisions in the LHC, physicists at CERN aim to study specific phenomena, such as quark-gluon plasma, a hot and dense state of matter thought to have existed for a few millionths of a second in the early Universe, shortly after the Big Bang.
The season of heavy-ion physics will come to an end on 30 October at 6 a.m. CET.
Join CERN on Thursday, 2 November, at 3 p.m. CET, live from the CERN Control Centre (CCC), where scientists from the LHC experiments and other experts will answer your questions about heavy-ion physics and the data they were able to collect this season.
Watch the video below showing the beginning of the lead-ion run at the LHC after 5 years.ckrishna Wed, 10/25/2023 - 11:48 Byline Bianca Moisa Publication Date Wed, 10/25/2023 - 10:23
Located near Geneva airport, the LHCb experiment is one of the four big experiments at CERN’s Large Hadron Collider (LHC). Dedicated to the study of b-quarks, LHCb uses a set of successive detectors to study the traces of the particles thrown forward from the collision point. One of these detectors is the outer tracker, which was replaced during Long Shutdown 2 by a new set-up based on scintillating fibers, the SciFi. The latter comes with a more refined granularity, allowing for a higher spatial resolution of the tracked particles.
The outer tracker was still in good shape and working perfectly, despite having been in operation for a decade. Therefore, after discussing the spare detector module at a conference with colleagues from the GSI Helmholtzzentrum für Schwerionenforschung, the LHCb collaboration decided to donate it to the PANDA (antiProton ANnihilation at DArmstadt) experiment, which will be hosted by FAIR – the Facility for Antiproton and Ion Research – currently under construction at GSI.
At PANDA, the outer tracker will partly go back to resume its initial function of tracing the smallest building blocks of matter. With the aid of the FAIR accelerators, antiproton beams will be generated and stored, then collided with fixed targets (e.g. hydrogen) inside the PANDA detector set-up. As this will happen at lower energies, the outer tracker will be able to detect the light hadrons produced by the collisions. Hadron spectroscopy is where the physics goals of LHCb and PANDA overlap, and the two will be able to collect complementary data that can later be analysed and compared. The tracker will also be used by students and young researchers in R&D projects, as well as in outreach activities for schools and the general public.
Transporting the tracker was no easy feat: In its transport frame, it is seven metres long, 3.5 metres wide and 5.5 metres high. It also weighs 24 tons. In 2018, when the disassembly started, the whole outer tracker was demounted placed in its transport frame – a specially designed handling cage – and removed from the LHCb cavern. Subsequently, it was moved to a storage hall within CERN, and more recently to Sergy, France for the release procedures and finally back to LHCb in Meyrin, Switzerland, where it was prepared for shipping. Hoisted up by cranes onto a truck, the detector began its journey from CERN to GSI/FAIR. Near Colmar, France, it was loaded onto a ship for a multi-day journey up the Rhine river. At Gernsheim, Germany, another truck collected the tracker and brought it safely to GSI/FAIR in Darmstadt where it will start its second life. Read more on the LHCb website here.Impressions of the outer tracker’s journey from LHC Point 8 at CERN to GSI/FAIR in Darmstadt, where it now awaits its second life.
The donation was made possible thanks to the close cooperation in logistics and technical aspects between several colleagues at CERN and GSI/FAIR, in particular Niels Tuning (LHCb, Nikhef/CERN) and Anastasios Belias (PANDA, GSI/FAIR) and their relentless efforts to give the formidable outer tracker a second life. The donation was kindly agreed upon by the LHCb groups who meticulously built and operated the outer tracker, namely,
The LHC is back delivering collisions to the experiments after the successful leak repair in August. But instead of protons, it is now the turn of lead ion beams to collide, marking the first heavy-ion run in 5 years. Compared to previous runs, the lead nuclei will be colliding with an increased energy of 5.36 TeV per nucleon (compared to 5.02 TeV per nucleon previously) and the collision rate has increased by a factor of 10. The primary physics goal of this run is the study of the elusive state of matter known as quark-gluon plasma, that is believed to have filled the Universe up to a millionth of a second after the Big Bang and can be recreated in the laboratory in heavy-ion collisions.
Quark-gluon plasma is a state of matter made of free quarks (particles that make up hadrons such as the proton and the neutron) and gluons (carriers of the strong interaction, which hold the quarks together inside the hadrons). In all but the most extreme conditions, quarks cannot exist individually and are bound inside hadrons. In heavy-ion collisions however, hundreds of protons and neutrons collide, forming a system with such density and temperature that the colliding nuclei melt together, and a tiny fireball of quark-gluon plasma forms, the hottest substance known to exist. Inside this fireball quarks and gluons can move around freely for a split-second, until the plasma expands and cools down, turning back into hadrons.Event display showing a lead-lead collision in the ATLAS detector. (Image: ATLAS)
The ongoing heavy-ion run is expected to bring significant advances in our understanding of quark-gluon plasma. In addition to the improved parameters of the lead-ion beams, significant upgrades have been performed in the experiments that detect and analyse the collisions. ALICE, the experiment which primarily focuses on studies of quark-gluon plasma, is now using an entirely new mode of data processing storing all collisions without selection, resulting in up to 100 times more collisions being recorded per second. In addition, its track reconstruction efficiency and precision have increased due to the installation of new subsystems and upgrades of existing ones. CMS and ATLAS have also upgraded their data acquisition, reconstruction and selection infrastructure to take advantage of the increased collision rates. ATLAS has installed improved Zero Degree Calorimeters, which are critical in event selection and provide new measurement capabilities. LHCb, in addition to performing studies of lead-lead collisions with an upgraded tracking system, is preparing a unique programme of fixed-target collisions of lead nuclei with other types of nuclei using its unique SMOG2 apparatus that allows various gases to be injected into the LHC collision area.Event display showing a lead-lead collision in the CMS detector. (Image: CMS)
Studies of quark-gluon plasma in this heavy-ion will focus on rare processes such as the production of heavy quarks, quarkonium states, real and virtual photons and heavy nuclear states. The increased number of collisions is expected to allow measurement of the temperature of the plasma using thermal radiation in the form of photons and electron-positron pairs. Hydrodynamic properties of the near-perfect liquid state of matter will be measured in greater detail and “tomography” using particles such as the charm or beauty quarks that are produced in the initial phase of the collision, pass through the plasma and are detected afterwards. All these measurements will be far more precise than before.
In addition to studies of quark-gluon plasma, the experiments will be looking at so-called ultra-peripheral collisions of heavy ions, in which the beams do not collide directly, but one beam emits a high-energy photon that strikes the other beam. These collisions will be used to probe gluonic matter inside nuclei and to study rare phenomena such as light-by-light scattering and τ lepton photoproduction.
Five years after the previous heavy-ion run, expectations are high.ptraczyk Tue, 09/26/2023 - 16:59 Byline Piotr Traczyk Publication Date Thu, 09/28/2023 - 09:29
Located at CERN’s North Area and receiving beams from the Super Proton Synchrotron (SPS), the NA64 and NA62 experiments search for dark matter, complementing searches at the LHC, as they cover a different energy range. Both experiments recently published new results.
Dark matter does not seem to interact with our visible world but makes up most of our Universe. Researchers assume that the dark sector interacts with the Standard Model via so-called mediators. These mediators could be, for instance, a dark photon, a dark scalar boson and an axion, which could be distinguished by how they interact with Standard Model particles.
The NA62 experiment, which was designed to study the ultra-rare kaon decay into a charged pion and two neutrinos, has now searched for possible contributions from dark-matter particles to another rare kaon decay. The researchers used a beam consisting mainly of pions and kaons, produced by firing the 400 GeV/c SPS proton beam onto a beryllium target. The rare kaon decay into a pion and a pair of photons, subsequently decaying into two electron–positron pairs, is particularly interesting as, hypothetically, dark bosons would decay into the same final states as Standard Model photons. Although the experimentalists did not find evidence for such a rare decay, nor for a dark boson, they placed the most stringent upper limits to date by analysing data recorded in 2017 and 2018. In addition, the experimentalists excluded the axion as a possible explanation for the 17 MeV/c2 ATOMKI anomaly and thus confirmed previous findings by the NA64 experiment.
The NA64 collaboration hunts for invisible light dark-matter particles that interact with Standard Model particles through a possible dark photon. Using electron collision data collected between 2016 and 2022, corresponding to 9.4 × 1011 electrons on target, NA64 started to probe the very exciting region of parameter space predicted by two benchmark dark-matter models for the first time. Their dataset excludes leading sub-GeV dark-matter candidates with a coupling between the dark-matter particle and the dark photon for a range of dark-matter particle masses, ruling out both models. To obtain these results, the NA64 experiment used a 100 GeV/c electron beam generated from protons interacting with a fixed target. The collaboration utilised an active beam dump and attempted to reconstruct the hypothetical dark photon, via both visible electron–positron pairs and missing energy for invisible decays.ckrishna Mon, 08/21/2023 - 14:01 Byline Kristiane Bernhard-Novotny Publication Date Mon, 08/21/2023 - 14:00