J/ψ production in p+p at s=500 GeV collisions and Au+Au collisions at sNN=200 GeV at the STAR experiment

In this paper, we present measurements of J/ψ cross-section and yield dependence on charged-particle multiplicity in p+p collisions at s=500 GeV at mid-rapidity in the transverse momentum range of 0-20 GeV/c. Measurements of J/ψ nuclear modification factors in Au+Au collisions at sNN=200 GeV up to 14 GeV/c via the di-muon channel with the full data sample taken during RHIC 2014 run is also presented.


Introduction
The study of J/ψ meson production has been a long-term interest both in elementary collisions and heavy-ion collisions. In p+p collisions, the study of J/ψ production provides test ground for calculations based on perturbative Quantum Chromodynamics (pQCD) and the evolution of produced cc pair. However, none of the existing theoretical models can fully describe J/ψ production. J/ψ measurements at a new beam energy of √ s = 500 GeV in p+p collisions will provide additional insights into its production mechanism. On the other hand, the production of J/ψ has also been extensively used to probe the Quark-Gluon Plasma (QGP) created in heavyion collisions [1]. The suppression of J/ψ in a deconfined medium due to Debye color screening of the charm quark potential was proposed as a signature of the QGP formation. Interpretation of the J/ψ suppression is, however, still a challenge due to the contributions from the regenerated J/ψ by the coalesced uncorrelated cc pairs in the medium and the cold nuclear matter effects. Precise measurements of the nuclear modification factor (R AA ) for J/ψ over a broad kinematic range in Au+Au collisions can help better understand the feature of Debye color screening. At STAR, the newly installed Muon Telescope Detector (MTD), which provides both the di-muon trigger and the muon identification capability at mid-rapidity, opens the door to measuring J/ψ via the di-muon decay channel for the first time at STAR. The STAR experiment recorded data corresponding to integrated luminosities of 28.3 pb −1 and 22 pb −1 for p+p collisions at √ s = 500 GeV in the RHIC 2013 and 2011 run, respectively, by using the MTD di-muon trigger and the Barrel Electromagnetic Calorimeter (BEMC) trigger, and 14.2 nb −1 for Au+Au collisions at √ s N N = 200 GeV in the RHIC 2014 run via the di-muon trigger. In this proceedings, we report 1) the measurements of J/ψ cross-section and yield dependence on event multiplicity in p+p collisions at √ s = 500 GeV; and 2) the measurements of J/ψ R AA in Au+Au collisions at √ s N N = 200 GeV.     x T scaling of J/ψ cross section scaled by the branching ratio B in the di-muon decay channel (blue star) and in the di-electron decay channel (red circle).

J/ψ measurements in p+p collisions at
√ s = 500 GeV 2.1. J/ψ transverse momentum spectrum and x T scaling Figure 1 shows the inclusive cross section of J/ψ in p+p collisions at √ s = 500 GeV measured via the di-electron and di-muon channels for the transverse momentum (p T ) range of 0 < p T < 20 GeV/c. The measurement via the di-muon decay channel extends p T reach down to 0. The results in these decay channels are consistent in the overlapping range of 4 < p T < 9 GeV/c. The experimental results can be well described by CGC+NRQCD calculations at low p T [3] and NLO NRQCD calculations at high p T [4]. Figure 2 shows the x T = 2p T / √ s scaling of J/ψ cross section [5], where the J/ψ cross section in p+p collisions at √ s = 500 GeV follows a common trend as a function of x T at high p T . The breaking of the x T scaling at low p T can be attributed to soft processes.

J/ψ yield versus event activity
At STAR, we use the number of tracks matched to the Time-of-Flight (TOF) detector in the pseudorapidity interval |η| < 1 to characterize event activity. The relative yields of J/ψ as a function of event activity from both the di-electron and di-muon channels are presented in Fig.  3. The high-p T J/ψ results (p T > 4 GeV/c) are from the di-electron channel using BEMCtriggered data. The low-p T J/ψ results (p T > 0 GeV/c) are from the di-muon channel using MTD-triggered data. A similar stronger-than-linear trend is observed both at LHC [6] and RHIC. Comparisons of STAR measurements to different model calculations are shown in Fig. 4 for different kinematic ranges. PYHTIA8 with a default setting, including the Multiple-Parton Interaction (MPI) contributions to particle production, can describe the rising trend and p T dependence in data. Percolation model [7] also qualitatively reproduces the trend in data.

J/ψ measurement in Au+Au collisions at
√ s N N = 200 GeV Figure 5 shows the J/ψ invariant yield as a function of p T for different centralities in Au+Au collisions at √ s N N = 200 GeV within |y| < 0.5 . The measurements from the di-muon channel are consistent with the published di-electron results [8,9] at the same collision energy within    |y| < 1. Figure 6 shows the J/ψ R AA in different centrality bins for Au+Au collisions. A strong suppression at low p T range in all centrality bins is observed. This suppression could be due to the effects of dissociation and cold nuclear matter (CNM). The rising trend as a function of p T can be a result of formation-time effects and more feed-down contribution from B hadrons at high p T . Figures 7 and 8 show the centrality dependence of J/ψ R AA at low-p T and high-p T ranges, separately. The smaller suppression at the LHC [10,11] in central collisions at low p T indicates larger regeneration contribution due to higher charm cross section, while the larger suppression of J/ψ at LHC at high p T indicates larger dissociation rate due to higher temperature of the medium or shadowing effects. Transport models from Tsinghua [12,13] and Texas A&M University [14,15], which include both dissociation and regeneration effects, are also shown. At low-p T range, both models can describe the centrality dependence at RHIC, but

Summary
We present J/ψ production in p+p collisions at √ s = 500 GeV and Au+Au collisions at √ s N N = 200 GV at RHIC. In p+p collisions, the inclusive J/ψ spectrum is measured down to p T = 0 and the spectra can be well described by CGC + NRQCD and NLO NRQCD calculations. The relative J/ψ yield grows rapidly as the charged-particle multiplicity increases and a strongerthan-linear rise at higher multiplicities for p T > 4 GeV/c is observed. PYTHIA8 and the percolation model can qualitatively describe the observed trend in data. In Au+Au collisions, we observe clear J/ψ suppression at p T < 5 GeV/c at all centralities and J/ψ R AA can be qualitatively described by transport models including dissociation and regeneration effects.