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
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
In the first week of December, a major milestone was reached: the Muon Forward Tracker (MFT) was installed in the ALICE experiment (LHC, CERN). This is a brand new trajectograph that will extend the physics programme associated with the muon spectrometer. The picture shows the MFT in place around the LHC beam tube. The detector is a 60cm diameter, 40cm long cone; collisions will take place 40cm in front of the detector. Subatech was responsible for and played a crucial role in several parts of the MFT: the flexible printed circuit boards (green stripes in the photo) that power, read and control the silicon pixel sensors, the entire mechanics of the detector core (the sensor support disks, the cooling system and the cone structure holding the disks) with the associated integration, the sensor power supply system and all the assembly steps.
With this crucial step behind us, the period of commissioning the detector and integrating it into the ALICE environment to make it work with the other detectors in the experiment begins. The first collisions of the LHC Run 3 are expected in February 2022. A big thank you to all the technical staff of the lab who have contributed to this project since 2011 and without whom the MFT could not have existed.
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)