The research program on Quark-Gluon Plasma (a deconfined state of nuclear matter) is on the verge of considerably extending with the restart of lead-lead collisions at the highest energy (= 5,36 TeV). On September 26th 2023, the LHC delivered lead beams with collision rates between 2 and 6 kHz; the goal is to reach quickly rates of about 50 kHz. This data taking period will stop at the end of October. During this period of LHC Run 3, the cumulated statistics will be 10-fold that of Run 1 (2010-2013) and Run 2 (2015-2018) all together. Those data are crucial for the ALICE experiment because they will allow to reach the core of the physics program on Quark-Gluon Plasma with a better precision but, above all, new observable thanks to the upgrades installed during the last long shutdown (2018-2022).
Display of a Pb-Pb collision on September 27th with the ALICE experiment, using different detectors and among them the Muon Spectrometer, the Muon Forward Tracker and the Electromagnetic Calorimeter.
Page collaboration ALICE : https://alice.cern/
Page Plasma team @ SUBATECH : http://www-subatech.in2p3.fr/en/research/research-team/plasma/about
Congratulations to Rita Sadek who won an ALICE Thesis Award !
Rita received her award during the ALICE collaboration meeting on July 12th, 2023
The prize rewards each year two PhD thesis defended in the previous year. The thesis are selected on the basis of the level of the results obtained, the quality of the manuscript and the importance of the contribution to the collaboration.
Thesis title :
MFT commissioning and preparation for Run 3 data analysis with ALICE (LHC, CERN)
defended on October 28th, 2022 in Nantes
Supervisor : Gines Martinez
Co-supervisors: Guillaume Batigne, Maxime Guilbaud
Link to the thesis : https://theses.hal.science/tel-04008085
ALICE collaboration : https://alice.cern/
Plasma team @ SUBATECH : http://www-subatech.in2p3.fr/en/research/research-team/plasma/about
After a restart in July 2022, the LHC again delivered Pb collisions at the record energy of √s_NN= 5.36 TeV for a few hours on 18 November 2022. This first test of Pb-Pb collisions is an important step for the ALICE experiment to optimise and prepare the data taking and compression for the Quarks and Gluons Plasma (deconfined state of nuclear matter) programme in the 2023 Pb-Pb Run.
View of a Pb-Pb collision on 18 November by the ALICE experiment in its various detectors including the Muon Spectrometer, the Muon Forward Tracker and the Electromagnetic Calorimeter
Web site ALICE collaboration : https://alice.cern/
Web site Plasma team @ Subatech : http://www-subatech.in2p3.fr/fr/recherche/equipes/plasma/presentation
The lead nuclei accelerated at LHC (CERN) at ultrarelativistic energies produce an important electromagnetic field as a photon field at a density of few photons around the nuclei. This field reaches record density of about 1024 photons per cm2, with photon quanta energies up to gigaelectronvolt. The LHC becomes a photon-photon or photon-hadron collider; the photon induced reactions are studied in ultraperipheral collisions at LHC (Fig 1) but also in collisions with nuclear overlap.
In 2016, the ALCE collaboration observed a production of J/Ψ (bound state of a charmed quark and antiquark pair) in peripheral Pb-Pb collisions at √SNN =2.76 TeV, at very low transverse momentum, much higher than theoretically predicted in hadronic processes. This result was the first indication of photo-production of J/Ψ in collisions with nuclear overlap. This was interpreted as a coherent interaction of a photon with the whole Pb nucleus producing a J/Ψ with same quantic numbers as the photon with a mass 3 times above the proton mass. The photon fluctuates into a quark-antiquark pair finally coupling to the nucleus to create a J/Ψ. The understanding of the coherent photo-production, the interaction with the whole nuclei that is further broken during the interaction with nuclear overlap, is challenging for the theory.
The ALICE collaboration measured this coherent production at forward rapidity (2.5<y<4) in the muonic decay channel of J/Ψ peripheral Pb-Pb collisions at √SNN = 5.02 TeV and for a first time a signal has been observed (> 5σ) in semi-central collisions. The coherent photoproduction of is presented on Figure 2 as a function of the mean number of participating nucleons <Npart> (in this representation the most central collisions correspond to a mean number of participating nucleons of <Npart> ≈400). Those results show a weak centralility dependence and an enhancement of the photoproduction with the collision energy. The results are reproduced by theoretical calculations used for describing photoproduction in ultraperipheral collisions but modified to take into account the constraints of the interaction with nuclear overlap. The future Run 3 measurement (with better statistics and precision) should allow to clarify theoretical models of coherent photoproduction but also should allow to study this process in more central collisions. This could test the photoproduced J/Ψ dissociation in the quark gluon plasma formed in the nuclear overlap volume.
Figure2 : J/Ψ photoproduction cross section as a function of mean number of nucleons participating in hadronic Pb-Pb interactions, at a center of mass energy √SNN = 5.02 TeV. Results are compared to theoretical predictions.
More informations : arXiv:2204.10684 à paraître dans Phys.Lett.B. & Courrier du CERN : Oct2022
 Phys. Rev. Lett. 116 no. 22, (2016) 222301, arXiV 1509.08802
Web site collaboration ALICE : https://alice.cern/
Web site team Plasma @ Subatech : http://www-subatech.in2p3.fr/fr/recherche/equipes/plasma/presentation
Ten year after the Higgs boson discovery* and after 3 ½ years of shutdown the LHC (Large Hadron Collider) at CERN restarts! July 5th at 16h47 the LHC delivered first proton -proton collisions at 13.6 TeV to the 4 experiments: ALICE, ATLAS, CMS, LHCb. ALICE experiment started its physics program with new detectors installed during shutdown, in particular the Muon Forward Tracker (MFT) for which the Subatech ALICE group had a major contribution. A new readout electronics, in order to support a greater luminosity, and a new reconstruction and analysis software will allow to study these collisions and continue the study of the fundamental properties of the universe.
ALICE Collaboration: First proton-proton collisions at 13,6 TeV in ALICE with among other detectors: MFT, muon spectrometer (MCH, MID) electromagnetic calorimeter (EMC).
Restart of LHC live webcast: https://www.youtube.com/watch?v=06kFq1QF5-s
ALICE collaboration webpage : https://alice.cern/
Plasma team ALICE@ Subatech webpage: http://wwwsubatech.in2p3.fr/fr/recherche/equipes/plasma/presentation
Link to the series that IN2P3 has put online since April on the LHC restart ( Episode 1 : https://www.in2p3.cnrs.fr/fr/cnrsinfo/au-cern-les-scientifiques-reprennent-l-exploration-de-l-univers)