mardi 28 avril 2015 à 16:00
Quantum ChromoDynamics is the theory associated to the strong interaction in the standard model. It predicts that partons (quarks and gluons) are confined into hadrons at standard thermodynamic conditions. A deconfined hadronic matter state, the Quark-Gluon Plasma (QGP), is predicted for a high energy density and would have existed in the early state of Universe. ALICE (A Large Ion Collider Experiment) at CERN-LHC (Large Hadron Collider) is dedicated to hadronic matter study in p-p, p-Pb (Pb-p) and Pb-Pb collisions where QGP formation is expected. Photons produced by different mechanisms (hard processes, fragmentation, decay), sensitive or not to the medium, are especially interesting to probe the matter.
The ones coming directly from parton-parton hard scatterings can be discriminated in the measurement using the isolation analysis method. In ALICE, for this study, EMCAL (Electromagnetic calorimeter) is the main detector, where electromagnetic particles lose their energy by combination of Bremsstrahlung and pair production. It covers a range of |η| < 0.7 in pseudorapidity and 1.4 < φ < 3.3 in azimuth. The isolated photon measurement in p-Pb collisions will enable to test the QCD theoretical predictions in cold nuclear matter. Moreover it will be a reference for further hot nuclear matter analysis in Pb-Pb collisions.