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Abstract

Charmonia are a key observable for deconfinement in nucleus-nucleus (AA) collisions at the LHC. Measurements in proton-nucleus (pA) collisions and their comparison with proton-proton (pp) collision results provide complementary information on nuclear effects that are present in the absence of the Quark-Gluon Plasma (QGP). In addition, the calculation of the production of charmonia in pA collisions presents a challenging test of perturbative QCD at low scales Q^2 and low fractional momentum of the involved partons.

The inclusive J/psi production in proton-lead collisions at sqrt(s_NN) = 5.02 TeV has been measured with ALICE in the ee-decay channel at mid-rapidity down to vanishing transverse momentum (p_T): integrated, as a function of pT, centrality and charged-particle multiplicity. The nuclear modification factor of J/psi indicates a strong suppression of charmonium production integrated, at low p_T and as function of centrality. The dependence of the normalised J/psi yield as a function of the normalised charged particle multiplicity is consistent with the measurement in pp collisions and in the open heavy-flavour sector. It provides information on the correlation of soft and hard particle production.

An extrapolation of the expected J/psi production behaviour in AA collisions based on the pA measurement assuming the factorisation of the nuclear modification strengthens the evidence for an additional J/psi production component at low p_T in AA collisions. This component is expected from J/psi production from unbound charm quarks within the QGP or at the phase boundary.