First ALICE results on quarkonium production at Run 2 energies

About

Quarkonium production in hadronic collisions (either proton-proton or heavy ions) has been extensively studied in both fixed target and collider experiments. It is understood as the production of a heavy quark pair (ccbar or bbar depending on the quarkonium state) in a hard scattering process which occurs early in the collision, followed by the evolution of this quark pair into a colorless bound state. While the production of the quark pair is reasonably well described by perturbative QCD calculations, its evolution into the bound state is inherently non-perturbative and is studied experimentally in pp collisions. In heavy ion collisions on the other hand, quarkonia are used to probe the properties of the medium formed in the collision and in particular that of the quark-gluon plasma, via competing mechanisms such as color screening, thermal dissociation or recombination, as well as so-called cold nuclear matter effects such as shadowing, gluon saturation or energy loss.

The first ALICE results on quarkonium production from the LHC Run 2 data taking will be discussed. We will present preliminary J/psi and psi(2S) cross sections measured at forward rapidity in pp collisions at a center of mass energy sqrt(s)=13 TeV, as well as recently published J/psi nuclear modification factors RAA in Pb-Pb collisions at sqrt(s_NN)=5 TeV. Preliminary Upsilon(1S) RAA will also be presented. ALICE results will be compared to measurements performed by other LHC experiments, to results obtained at lower center of mass energies and to theoretical calculations. .

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