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11 March 2019

A report from the ALICE experiment

The study of the production of quarkonia, the bound states of heavy quark–antiquark pairs, is an important goal of the ALICE physics programme. The quarkonium yield is suppressed in heavy-ion collisions when compared with proton–proton collisions because the binding force is screened by the hot and dense medium. This suppression is expected to be greatest for events with high “centrality”, when the heavy ions collide head-on.

The ALICE collaboration has recently analysed the suppression of inclusive bottomonium (bb̅) production in lead-lead collisions relative to proton–proton collisions. This reduction is quantified in terms of the nuclear modification factor RAA, which is the ratio of the measured yield in lead-lead to proton–proton collisions corrected by the number of binary nucleon–nucleon collisions. An RAA value of unity would indicate no suppression whereas zero indicates full suppression. The bottomonium states ϒ(1S) and ϒ(2S) were measured via their decays to muon pairs at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV, in the rapidity range 2.5 < η< 4, with a maximum transverse momentum of 15 GeV/c. No significant variation of RAA is observed as a function of transverse momentum and rapidity, however, production is suppressed with increasing centrality (figure 1). A decrease in RAA from 0.60±0.10(stat)±0.04(syst) for the peripheral 50–90% of collisions to 0.34±0.03(stat)±0.02(syst) for the 0–10% most central collisions was observed.

Theoretical models must deal with the competing effects of melting and (re)generation of the ϒ within the quark-gluon plasma,  the shadowing of parton densities in the initial state and “feed-down” from higher resonance states. Due to uncertainties on the parton density, is not yet known whether the direct production of ϒ(1S) is suppressed, or merely the feed-down from ϒ(2S) and other higher-mass states. Nevertheless, the precision of these measurements imposes significant new constraints on the modelling of ϒ production in lead-lead collisions.