J/\psi measurements in the STAR experiment

J/ψ measurements in the STAR experiment

Barbara Trzeciak(for the STAR Collaboration)
Abstract

In this paper, we present recent STAR J/ results. J/ nuclear modification factors () in Au+Au collisions at 200, 62.4 and 39 GeV and in U+U collisions at 193 GeV are measured and compared to different theoretical calculations. We also report J/ elliptic flow () results in Au+Au collisions at 200 GeV and the first to J/ ratio measurement in collisions at 500 GeV.

1 Introduction

It was proposed that quarkonia are dissociated in the hot medium due to the Debye screening of the quark-antiquark potential and thus this ”melting” can be a signature of Quark-Gluon Plasma (QGP) formation [1]. But there are other mechanisms that can alter quarkonium yields in heavy-ion collisions relative to collisions, for example statistical recombination of heavy quark-antiquark pairs in the QGP or cold nuclear matter (CNM) effects. Systematic measurements of the quarkonium production for different colliding systems, centralities and collision energies may help to understand the quarkonium production mechanisms in heavy-ion collisions as well as the medium properties.

2 J/ψ and ψ(2S) measurements

STAR has measured J/ spectra [11, 7] and polarization [12] in collisions at 200 GeV via the dielectron decay channel ( 5.9%) at mid-rapidity ( 1). These results are compared to different model predictions to understand J/ production mechanism in elementary collisions. In order to further test the charmonium production mechanism and constrain the feed-down contribution from the excited states to the inclusive J/ production, the J/ and signals were extracted in collisions at 500 GeV. Figure 1 shows ratio from STAR (red full circle) compared to measurements of other experiments at different colliding energies, in and A collisions. The STAR data point is consistent with the observed trend, and no collision energy dependence of the to J ratio is seen with current precision.

In Au+Au collisions at 200 GeV STAR has measured J/ spectra for different centrality bins [8, 7]. It was found that at low ( 2 GeV/) the J/ spectra are softer than the Tsallis Blast-Wave prediction, assuming that J/ flows like lighter hadrons [8]. This suggests that recombination may contribute to low- J/ production. Measurement of J/ may provide additional information about the J/ production mechanisms. Figure 2 shows J/ measured in STAR in Au+Au collisions at = 200 GeV [2]. At 2 GeV/ is consistent with zero. Compared to different model predictions [3, 4, 5, 6], data disfavor the scenario that J/ with 2 GeV/ are dominantly produced by recombination (coalescence) from thermalized pairs. Figure 3 shows J/ as a function of the number of participant nucleons () in Au+Au collisions at = 200 GeV, separately for low- ( 5 GeV/[8] and high- ( 5 GeV/[7] regions. Suppression increases with collision centrality and the at high is systematic higher than the low- one. The strong suppression of high- J/ observed in central collisions (0-30%) indicates color screening or other QGP effects – at 5 GeV/ J/ are expected to be less affected by the recombination and CNM effects. The results are compared with two models, Zhao and Rapp [9] and Liu et al. [10]. Both models take into account direct J/ production with the color screening effect and J/ produced via the recombination of and quarks. The Zhao and Rapp model also includes the J/ formation time effect and the B-hadron feed-down contribution. At low both predictions are in agreement with the data, while the high- result is well described by the Liu et al. model and the model of Zhao and Rapp underpredicts the measured .

Low- J/ measurements in Au+Au collisions at various colliding energies: 200 (black), 62.4 (red) and 39 (blue) GeV are shown in Fig. 4. Within the uncertainties, a similar level of suppression is observed for all three energies, which can be described by the model predictions of Zhao and Rapp [9]. However, it should be noted that due to lack of precise measurements at 62.4 and 39 GeV Color Evaporation Model calculations [13] are used as baselines, which introduce large uncertainties. Figure 4 also shows the Minimum Bias measurement in U+U collisions at 193 GeV as a full circle. In U+U collisions one can reach up to 20% higher energy density compared to Au+Au collisions in the same centrality bin [14]. No difference in suppression compared to other measurements presented in Fig. 4 is observed.

3 Summary

In summary, significant suppression of low J/ is seen in Au+Au collisions at various colliding energies: = 200, 62.4 and 39 GeV, and in U+U collisions at 193 GeV. No strong energy dependence of the J/ suppression in Au+Au is observed. Also, high- J/ in Au+Au collisions at = 200 GeV are strongly suppressed in central collisions, which suggests the QGP formation. to J/ ratio was measured for the first time in collisions at 500 GeV. When compared to results from other experiments, no collision energy dependence of the ratio is seen.

Acknowledgements

This publication was supported by the European social fund within the framework of realizing the project ,,Support of inter-sectoral mobility and quality enhancement of research teams at Czech Technical University in Prague”, CZ.1.07/2.3.00/30.0034.

References

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