




Le Large Hadron Collider (LHC) au CERN a récemment mené à bien le programme de collisions de protons sur des noyaux de plomb. C’est à dessein que cette catégorie de collisions a été choisie, car elle permet d’étudier comment l’environnement nucléaire perturbe la structure interne d’un nucléon individuel donné dans le noyau de plomb. Ces effets nucléaires dits « froids » ont déjà été étudiés dans le passé. La grande nouveauté vient du bond en énergie du LHC qui était très attendu. Il permet en effet de tester notre compréhension à très haute énergie de ces effets. Une particule-sonde, le J/psi, a été privilégiée par les expériences ALICE et LHCb installées auprès du LHC. La production du J/psi y a été mesurée et est confrontée avec succès à l’extrapolation à haute énergie de notre modèle théorique. Le SPhN a activement contribué à la fois à la mesure du taux de production du J/psi en collisions proton-plomb et à l’élaboration de ce modèle.

The first lead-lead collision results have been published

After almost a year collecting data from proton-proton collisions, the LHC at CERN began the injection of lead ions at the beginning of November, with the first collisions obtained on November 8. The energy in the nucleon-nucleon center of mass is 2.76 TeV, around ten times greater than that achieved previously by the RHIC in Brookhaven USA. The first results from ALICE have been made available without delay.

The Phenix and Star collaborations, which include physicists from CEA-IRFU and CNRS-IN2P3, have announced major discoveries on the nature of the quark-gluon plasma. These conclusive results, which advance our understanding of nuclear material subjected to extreme conditions, shed new light on the birth of the universe. They have been published in the journal Physical Review Letters.


Monday, 23 November 2009, marked the first particle beam collision inside the large detectors of the LHC. ALICE saw its first collisions at an energy of 900 GeV, enabling it to check for correct operation of the 18 large detectors which comprise it. Since 27 November, with just a few days worth of data, the collaboration has even published an article confirming some existing measurements.

The group from the IRFU, who are responsible for the dimuon arms, had to await more stable beam conditions in order to see their detectors reacting to the data coming from the collisions and, on 6 December, all the gaseous detectors were able to be powered up. The traces from the first muons could be successfully reconstructed, to the great satisfaction to all the team. They are now eager to begin collecting more data after the planned restart in February 2010.

At the end of March 2009, the ALICE Muon Spectrometer took cosmic rays over a period of two weeks. The ALICE group at Saclay2 was closely involved in the design, development, construction and installation of a part of the chambers of this Spectrometer3. The purpose of the cosmic ray test was to check the performance of the entire system, from acquisition to reconstruction of the data. The acquisition system readout about a million channels and the data was recorded on the computing grid. Almost 15,000 tracks were reconstructed under conditions close to those of the real experiment. The cosmic test was a success, demonstrating the performance and the stability of the spectrometer chambers. It also revealed some weak points of the system. Tests are currently underway to correct the observed defaults.

Another cosmic test is planned for August, a few weeks before the first LHC beam injections, involving all ALICE detectors.







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