Production of $J/\psi$ and ϒ mesons in proton–lead collisions at $\sqrt{s_{NN}}=5.02\text{TeV}$

Abstract

The production of $J/\psi$ and ϒ-mesons decaying into the dimuon final state is studied at the LHCb experiment, covering the rapidity intervals $1.5<y<4.0$ and $-5.0<y<-2.5$ and transverse momentum $p_{\mathrm{T}}<15\text{GeV}/c$, in proton–lead collisions at a proton–nucleon centre-of-mass energy of $\sqrt{s_{NN}}=5.02\text{TeV}$. The analysis is based on a data sample corresponding to an integrated luminosity of $1.6{\text{nb}}^{-1}$. The nuclear modification factor and the forward–backward production ratio are determined for $J/\psi$ and $\Upsilon (1S)$ mesons. At large rapidity a clear suppression of prompt $J/\psi$ production is observed with respect to the production in pp collisions, while the suppression of $J/\psi$ from b-hadron decays is less pronounced. The nuclear modification factor for $\Upsilon (1S)$ mesons in the forward region is found to be similar to the one for $J/\psi$ from b-hadron decays.

Introduction

In ultra-relativistic heavy-ion collisions, the production of heavy quarkonia is expected to be suppressed with respect to proton–proton collisions, if a quark–gluon plasma (QGP) is created [1]. This suppression is considered as a probe sensitive to the entire evolution of the fireball evolution, since pairs of heavy quarks and antiquarks are produced at the early stage of heavy-ion collisions. On the other hand, cold nuclear matter (CNM) effects, such as nuclear shadowing and initial state partonic energy loss, will also cause a similar suppression. CNM effects can be disentangled using the measurement of quarkonium production in proton–nucleus ($p\mathrm{A}$) collisions. Therefore, it is important to study CNM effects in $p\mathrm{A}$ collisions for a quantitative understanding of the properties of the QGP.

In early 2013, the LHCb detector [2] collected an integrated luminosity of $1.6{\text{nb}}^{-1}$ of pPb collisions. The centre-of-mass energy of the nucleon–nucleon system is $\sqrt{s_{NN}}=5.02\text{TeV}$. During the data taking, the LHC swapped the directions of the proton and lead beams, which makes it possible to study the physics in both the forward region and the backward region, where “forward” means the proton beam direction, thus corresponding to positive rapidity y. With these data samples LHCb studied the productions of $J/\psi$ and ϒ-mesons using the dimuon final state [3], [4].