Abstract
The production of ϒ(nS) mesons (n = 1, 2, 3) in pPb and Pbp collisions at a centre-of-mass energy per nucleon pair sqrt{s_{mathrm{NN}}}=8.16 TeV is measured by the LHCb experiment, using a data sample corresponding to an integrated luminosity of 31.8 nb−1. The ϒ(nS) mesons are reconstructed through their decays into two opposite-sign muons. The measurements comprise the differential production cross-sections of the ϒ(1S) and ϒ(2S) states, their forward-to-backward ratios and nuclear modification factors. The measurements are performed as a function of the transverse momentum pT and rapidity in the nucleon-nucleon centre-of-mass frame y* of the ϒ(nS) states, in the kinematic range pT < 25 GeV/c and 1.5 < y* < 4.0 (−5.0 < y* < −2.5) for pPb (Pbp) collisions. In addition, production cross-sections for ϒ(3S) are measured integrated over phase space and the production ratios between all three ϒ(nS) states are determined. Suppression for bottomonium in proton-lead collisions is observed, which is particularly visible in the ratios. The results are compared to theoretical models.
Highlights
In pA collisions, this suppression is caused by nuclear phenomena unrelated to deconfinement, commonly called cold nuclear matter (CNM) effects
The CNM effects that are expected to affect quarkonia production are of two types, “initial-state” effects happening at a early stage of the collision, such as nuclear effects on parton densities [4,5,6,7] or coherent energy losses [8,9,10], and “final-state” effects, as quarkonia absorption by nucleons [11], expected to be negligible at LHC energies [12,13,14,15]
Another final-state effect is the breaking of the qqpair caused by collisions with comoving particles with similar rapidities, whose density is determined from the particle multiplicity measured in that region of rapidity
Summary
The LHCb detector [30, 31] is a single-arm forward spectrometer designed for the study of particles containing b or c quarks. The storage of only the triggered candidates enables a reduction of the event size by an order of magnitude For this analysis, at least one muon with pT > 500 MeV/c is required at the hardware trigger stage and at the software trigger stage, two muon tracks with pT > 300 MeV/c and a high-quality reconstructed decay vertex are required to form an Υ (nS) candidate with invariant mass m(μ+μ−) > 4.7 GeV/c2. Since the energy per nucleon in the proton beam is significantly larger than that in the lead beam, the proton-nucleon centre-of-mass system has a rapidity in the laboratory frame of +0.465 (−0.465) for pPb (Pbp) collisions, resulting in a shift of the effective detector acceptance In this analysis, Υ (nS) mesons are measured in the kinematic range of pT < 25 GeV/c, and 1.5 < y∗ < 4.0 for pPb forward and −5.0 < y∗ < −2.5 for pPb backward collisions. The luminosities are determined using van der Meer scans [47], which were performed for both beam configurations
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