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 extremely high energy densities obtained during lead-lead collisions may result in the formation of a short-lived extremely dense and hot state of matter similar to that which probably existed during the first few moments after the creation of the Universe. Quantum Chromodynamics, the fundamental theory underlying the strong interaction, predicts a phase transition between ordinary nuclear matter and this new state, in which the quarks and gluons are unconfined, when the density reaches several times that of ordinary nuclear matter and the temperature reaches around 2 million million degrees. The ALICE experiment in the LHC has been designed specifically to study this quark-gluon plasma (QGP). All the detectors are now operational, including the muon spectrometer to which IRFU has contributed. The above figure shows the large number of tracks generated during a typical collision. The data collection phase will continue until December 6.