Quarkonium measurements via the di-muon decay channel in p+p and Au+Au collisions with the STAR experiment

We present the rst J/ψ and measurements in the di-muon decay channel at mid-rapidity at RHIC using the newly installed Muon Telescope Detector. In p+p collisions at s=500 Gev, inclusive J/ψ cross section can be described by CGC+NRQCD and NLO NRQCD model calculations for 0 < pT < 20 GeV/c. In Au+Au collisions at sNN=200 Gev, we observe (i) clear J/ψ suppression indicating dissociation; (ii) J/ψ RAA can be qualitatively described by transport models including dissociation and regeneration with a tension at high pT; and (iii) hint of less melting of ϒ(2S + 3S) relative to ϒ(1S) at RHIC compared to that at LHC.


Introduction
Quarkonia are an essential probe to study the properties of the Quark Gluon Plasma (QGP). The suppression of J/ψ due to color-screening effects in the medium was initially proposed as a direct evidence of the QGP formation [1]. However, the interpretation of the J/ψ suppression is still a challenge due to the contributions from the regenerated J/ψ by the recombination of cc pairs in the medium and the cold nuclear matter effects. Therefore it is important to have more precise J/ψ measurements over a broad kinematic range and even cleaner Υ state measurements. The latter do not suffer from the regeneration contribution due to the much smaller bb pair cross section, i.e. σ bb ∼ 2 µb [2] while σ cc ∼ 800 µb [3] at top RHIC energy. 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 quarkonia via the di-muon decay channel at STAR. Compared to the di-electron decay channel, the dimuon decay channel suffers much less from bremsstrahlung and thus provides much better invariant mass resolution to separate different Υ states. Using the MTD di-muon trigger, the STAR experiment recorded data corresponding to an integrated luminosity of 28.3 pb −1 in p+p collisions at √ s = 500 GeV in the RHIC 2013 run, and 14.2 nb −1 in Au+Au collisions at √ s N N = 200 GeV in the RHIC 2014 run. In these proceedings, we report (1) the measurements of J/ψ production in p+p collisions at √ s = 500 GeV; and (2) the measurements of the nuclear modification factor (R AA ) for J/ψ and the production of different Υ states 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 dimuon decay channel (blue star) and in the di-electron decay channel (red circle).

J/ψ measurements in p+p collisions at
√ s = 500 GeV Figure 1 shows the cross section of J/ψ in p+p collisions at √ s = 500 GeV in the di-electron and di-muon decay channels for 0 < p T < 20 GeV/c. The di-muon decay channel extends p T reach down to 0 GeV/c. The results in these decay channels are consistent in the overlapping p T range of 4 < p T < 9 GeV/c. The experimental results can be well described by CGC+NRQCD calculations at low p T [4] and NLO NRQCD calculations at high p T [5]. Figure 2 shows the x T = 2p T / √ s scaling of J/ψ cross section [6]. The J/ψ cross section in p+p collisions at √ s = 500 GeV follows the 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 the soft processes.

J/ψ measurements in Au+Au collisions at
√ s N N = 200 GeV Figure 3 shows the invariant yield of J/ψ in Au+Au collisions at √ s N N = 200 GeV for different collision centralities. The new results in the di-muon decay channel are consistent with previous results in the di-electron decay channel [7,8] within uncertainties.
The nuclear modification factor R AA = σ inel Fig. 4, compared with LHC results [13,14]. The strong suppression at RHIC at low p T indicates that dissociation plays a significant role in this p T range. The hint of the increasing trend of R AA at RHIC at high p T can be explained by formation-time effects and feed-down of B hadrons. The less suppression of J/ψ at LHC at low p T indicates larger regeneration contribution due to higher charm cross section, while more suppression of J/ψ at LHC at high p T indicates larger dissociation rate due to higher temperature of the medium. Transport Models from Tsinghua [9,10] and Texas A&M University (TAMU) [11,12], including dissociation and regeneration effects, can qualitatively describe the p T dependence of RHIC and LHC data.
Centrality dependence of J/ψ cross section is shown in Fig. 5 for integrated p T and in Fig. 6 for p T > 5 GeV/c. For integrated p T , both models can describe centrality dependence at RHIC, but tend to overestimate suppression at LHC. For p T > 5 GeV/c, there is tension among models  and data. New measurements in the di-muon decay channel provide a distinguishing power for these transport models.

Υ measurements in Au+Au collisions at
√ s N N = 200 GeV Figure 7 shows the di-muon mass spectrum in Υ state mass range in Au+Au collisions at √ s N N = 200 GeV. We observe signs of an indication of Υ(2S + 3S) signals in the di-muon decay channel. The raw yields of Υ states are obtained by a simultaneous fit to the like-sign and unlike-sign distributions. In this fit, (i) the Υ state masses are fixed to the PDG values and their widths are determined by simulation; (ii) the ratio of Υ(2S)/Υ(3S) is fixed to the value in p+p collisions; and (iii) the shape of bb and Drell-Yan background is estimated using PYTHIA. Figure 8 shows the fitted Υ(2S + 3S)/Υ(1S) ratio compared with the world-wide p+p data [16] and CMS data [17,18]. There is a hint of less melting of Υ(2S + 3S) relative to Υ(1S) at RHIC than at LHC.

Summary and Outlook
We present the first J/ψ and Υ measurements in the di-muon decay channel at mid-rapidity at RHIC. In p+p collisions at √ s = 500 GeV, inclusive J/ψ cross section can be described by CGC+NRQCD and NLO NRQCD model calculations for 0 < p T < 20 GeV/c. In Au+Au collisions at √ s N N = 200 GeV, we observe (i) clear J/ψ suppression indicating dissociation; (ii) J/ψ R AA can be qualitatively described by transport models including dissociation and regeneration despite a tension at high p T ; and (iii) there is a hint of less melting of Υ(2S + 3S) relative to Υ(1S) at RHIC compared to that at LHC. These measurements in Au+Au collisions will have better statistical precision by combining the similar amount of data recorded in the RHIC 2016 run.