J/psi production in proton-nucleus collisions at ALICE: cold nuclear matter really matters

Heavy quarkonia are expected to be sensitive to the properties of strongly interacting matter, at both low and high temperatures. In nucleus-nucleus collisions, a phase transition to a deconfined state of quarks and gluons (Quark-Gluon Plasma) is thought to take place once the temperature of the system exceeds a critical temperature of the order of 150-200 MeV. The deconfined state can induce a suppression of charmonium (due to color screening, dominant at SPS and RHIC energies), which can be overturned at LHC energy by the (re)combination of the large number of free c and cbar quarks, taking place when the system cools down below the critical temperature. Cold nuclear matter also has an influence on heavy quarkonia. Such effects can be studied in proton-nucleus collisions, where no deconfined state is expected to be created. At LHC energy, they mainly include nuclear shadowing, gluon saturation, break-up of the quarkonium states, and parton energy loss in the initial and final state. The study of these effects gives important insights on perturbative and non-perturbative aspects of QCD. In addition, the presence of cold nuclear matter is known to play a role also in nucleus-nucleus collisions. Understanding this role is therefore crucial for correctly identifying and calibrating the influence of non-QGP related effects on the Pb-Pb collisions studied at LHC. In this seminar, after a short introduction on the main concepts and on results from lower energy experiments, the first results on J/psi production in p-Pb collisions at sqrt(s_NN) = 5.02 TeV from the ALICE experiment will be discussed. They include the study of the nuclear modification factors R_pPb at forward and backward rapidity, down to zero transverse momentum, as well as of the ratio of the forward and backward J/psi yields, differentially in y and p_T. The results will be compared to theoretical models and to preliminary results from other LHC experiments