Institut de Recherche sur les Lois Fondamentales de l'Univers

Objectives

According to the Quantum Chromodynamics (QCD), the fundamental theory of the strong interaction, a phase transition between the ordinary nuclear matter and a Quark Gluon Plasma (QGP) should occur for a high energy density (ε ~ 1 GeV/fm³) or temperature (T ~ 200 MeV). In the QGP, the elementary components of matter, the quarks and gluons, are no more confi ned inside the hadrons and are moving freely. The conditions of energy density and temperature needed are reached in the most central Pb-Pb collisions at the LHC, where the energy available in the nucleon-nucleon center of mass is √ s_NN = 5.5 TeV. This energy is ~25 times larger than in previous experiments at the RHIC collider. The SPhN group is focused in the study of heavy resonances (ϒ and J/ψ) screening as a signature of the QGP. The resonances are detected through the dimuon decay channel, where the muons are reconstructed in the forward Muon arm.

Simulations

An intensive work has been done during the last years to develop the ALICE simulation framework AliRoot which is continuously evolving to become also the reconstruction and analysis program. The SPhN group has played an important role and participates in particular in the improvement of the response of pad chambers of the Muon arm and in the alignment program (see below). An important contribution was also the evaluation of the sensitivity of the Muon arm to the ratio of ϒʼ/ϒ as a function of p_T. This ratio is considered as a signature that should discriminate between the QGP and some other scenarios. By using this ratio it is expected that the effect of the gluon shadowing will cancel out as well as many experimental systematic errors. The value in pp collisions of the ϒʼ/ϒ ratio as a function of p_T has been found to be constant by the CDF collaboration at the Tevatron. Therefore, a distortion of this ratio should be an indication of QGP formation. We have also contributed to develop a mixing event technique in view of the combinatorial background subtraction. This technique is particularly useful for the most central events in Pb-Pb collisions. 

 Localization:

The detector will be settled in the point 2 of the LHC collider at Cern (Geneva)

 Contribution of the Dapnia:

The dimuon arm

Contacts :