Inclusive, prompt and non-prompt J/ψ production at mid-rapidity in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Abstract

Abstract:The transverse momentum (p T ) dependence of the nuclear modification factor R AA and the centrality dependence of the average transverse momentum p T for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at √ s NN = 2.76 TeV in the e + e − decay channel at mid-rapidity (|y| < 0.8). The p T is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of R AA is observed towards low p T . These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, f B , has been determined in the region 1.5 < p T < 10 GeV/c in three centrality intervals. No significant centrality dependence of f B is observed. Finally, the R AA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5 < p T < 10 GeV/c is found to be stronger than predicted by most models. The ALICE collaboration 26 IntroductionHeavy-ion collisions at high energies allow the study of strongly interacting matter under extreme conditions. Calculations based on Quantum-Chromo-Dynamics (QCD) on the lattice indicate that the hot and dense medium created in these collisions behaves like a strongly coupled Quark-Gluon Plasma (QGP) [1][2][3][4]. Heavy quarks are an important probe for the properties of this state of matter, since they are produced via hard partonic collisions at a very early stage and thus experience the complete evolution of the system. Quarkonium states, i.e. bound states of a heavy quark and anti-quark such as the J/ψ meson (cc state) are of particular interest. It was predicted that the J/ψ formation is suppressed in a QGP due to the screening of the cc potential in the presence of free colour charges [5]. Experimentally, a suppression of the inclusive J/ψ yield in heavy-ion collisions relative to the corresponding yield in pp, scaled by the number of binary nucleon-nucleon collisions, has been observed at the Super Proton Synchrotron (SPS) [6][7][8] and the Relativistic Heavy Ion Collider (RHIC) [9,10]. The level of suppression was found to be similar at SPS and RHIC, despite the significantly different collision energy. More recently, the nuclear modification of J/ψ was also measured for Pb-Pb collisions at the LHC [11][12][13]. While at high transverse momentum (p T > 4 GeV/c) the suppression factor is at the same level as the one observed at RHIC in the low p T region, a significant reduction of the suppression is measured towards lower p T . This has been interpreted as the effect of an additional contribution to J/ψ production at low p T , due to the combination of correlated or uncorrelated c and c quarks [14,15]. This contribution becomes sizable at LHC energies, since the number of cc pairs is much higher than at lower energies. Assuming that a deconfined phase is produced and that all the J/ψ are dissociated, this process happens at the chemical freezeout stage of the fireball ...

Measurement of electrons from beauty-hadron decays in p-Pb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ TeV and Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

“…In the p T interval 3 < p T < 6 GeV/c, the suppression of the R PbPb for electrons from beauty-hadron decays is about 1.2σ less. This difference is consistent with the ordering of charm and beauty suppression seen in the prompt D meson and J/ψ from B meson comparison [34,40,41].Within uncertainties, the R pPb is described by pQCD calculations including modifications of the parton distribution functions (FONLL [29-31] + EPS09NLO [90] nuclear PDFs) as shown in figure 7 (left). The data and the calculation suggest that cold nuclear matter effects are small at high transverse momentum.…”

“…the parton p T spectrum and the fragmentation into hadrons [13,39]. Furthermore, the nuclear modification factors R PbPb of prompt D mesons and of J/ψ from B meson decays were compared in the p T interval 8 < p T < 16 GeV/c for D mesons and 6.5 < p T < 30 GeV/c for J/ψ mesons in order to have a similar average p T (≈10 GeV/c) for the heavy hadrons [34,40,41]. This comparison with models indicates that charm quarks lose more energy than beauty quarks in this p T interval in central Pb-Pb collisions.…”

Prospects for Quarkonium Measurements in p–A and A–A Collisions at the LHC

“…In the charmonium sector, ALICE presented a clear sign of a low transverse momentum (p T ) J/ψ production component leading to a larger nuclear modification factor R AA [7,8,9,10,12], i.e., a weaker suppression compared to the results at RHIC by PHENIX [13] and STAR [14,15] for the p T integrated results and at low p T . This increase has been widely interpreted as a signature of late-stage J/ψ production either during the life-time of the deconfined medium produced in heavy-ion collisions proposed in Ref.…”

Charmonium production in Pb–Pb collisions with ALICE at the LHC

“…The fraction of non-prompt to inclusive J/ψ yields, f b , is shown in Fig. 3 as a function of N part for 1.5 < p T < 10 GeV/c (left) and as a function of p T for 0 − 50 % collision centralities (right) [12]. No significant dependence on centrality is observed.…”

“…Ratio of non-prompt to inclusive J/ψ yields measured at mid-rapidity (|y| < 0.8), as a function of N part (left) and p T (right). Results are compared to similar measurements by CMS in Pb − Pb collisions as well as by ALICE, ATLAS, CMS and CDF in pp collisions [12].…”