ϒ suppression at forward rapidity in Pb–Pb collisions at sNN=5.02TeV

Received 23 May 2018 Received in revised form 28 October 2018 Accepted 12 November 2018 Available online 29 December 2018 Editor: M. Doser 1. Introduction A detailed study of the properties of the Quark–Gluon Plasma (QGP) [1] is the main goal of heavy-ion experiments at ultra- relativistic energies [2–6]. Quarkonia, i.e. bound states of charm or bottom quark–antiquark pairs, are sensitive probes of color decon- finement, due to the Quantum-Chromo Dynamics Debye screening mechanism [7–9] leading to quarkonium suppression. Moreover, the various quarkonium states have different binding energies and therefore different dissociation temperatures in a QGP, leading to sequential suppression [7,10]. Theory estimates [11] indicate that bottomonium formation may occur before QGP thermalization [12] because of the large bottom quark mass. In this situation, a quan- titative description of the influence of the medium on the bound states becomes challenging. While the dissociation temperatures vary significantly between different models [8,9], it is commonly accepted that the widths of the spectral functions of the bottomo- nium states increase compared to the widths in vacuum, due to the high temperature of the surrounding medium [13]. Finally, taking into account that feed-down processes from higher-mass resonances (around 40% for the Υ(1S) and 30% for the Υ(2S) [9]) are not negligible, the evaluation of the medium temperature via bottomonium measurements remains a complex endeavour. The first studies of quarkonium production in heavy-ion colli- sions were devoted to charmonium states, and a suppression of their yields was observed at the SPS [14–16], at RHIC [17,18] and ⋆ E-mail address: alice-publications@cern.ch. abstract Inclusive Υ(1S) and Υ(2S) production have been measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair √sNN = 5.02 TeV, using the ALICE detector at the CERN LHC. The Υ mesons are reconstructed in the centre-of-mass rapidity interval 2.5 < y < 4 and in the transverse- momentum range pT < 15 GeV/c, via their decays to muon pairs. In this Letter, we present results on the inclusive Υ(1S) nuclear modification factor RAA as a function of collision centrality, transverse momentum and rapidity. The Υ(1S) and Υ(2S) RAA, integrated over the centrality range 0–90%, are 0.37± 0.02(stat) ± 0.03(syst) and 0.10 ± 0.04(stat) ± 0.02(syst), respectively, leading to a ratio RΥ(2S)/RΥ(1S) of AA AA 0.28±0.12(stat)±0.06(syst). The observed Υ(1S) suppression increases with the centrality of the collision and no significant variation is observed as a function of transverse momentum and rapidity.