Measurements of ψ(2S) production in Pb–Pb collisions with ALICE at the LHC

Abstract

Quantum chromodynamics predicts the existence of dense and hot nuclear matter which is described in terms of a deconfined medium of quarks and gluons, known as quark-gluon-plasma (QGP). High energy density and temperature can be reached by colliding heavy-ions at ultrarelativistic energies, enabling the study of the QGP in the laboratory. The ALICE detector at the LHC was designed to study the properties of such a deconfined medium. In the presence of a QGP, the charmonium yield would be suppressed due to color Debye screening and dissociation. Due to its larger size and weaker binding energy, the (2S) is expected to experience a stronger suppression compared to the / . At the LHC, the magnitude of the / suppression is smaller than that observed at lower energies at SPS and RHIC, indicating that charmonium (re)generation via the (re)combination of charm and anticharm-quarks, happening either in medium or at the phase boundary, plays an important role at LHC energies. The (2S) production relative to / represents one possible discriminator between the two different regeneration scenarios. Due to its smaller production cross section and branching ratio in the dilepton decay channel, the (2S) measurement is more challenging as compared to the / one. The combined Run 2 data sets of ALICE allows one to extract the (2S) signal over the full centrality range, in Pb-Pb collisions at √ NN = 5.02 TeV and at forward rapidity with the muon spectrometer. In this contribution, we report on the (2S) nuclear modification factor and on the (2S)-to-/ single and double ratio, in Pb-Pb collisions at √ NN = 5.02 TeV, as a function of centrality and transverse momentum. All the measurements are compared with theoretical predictions