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We report on measurements of sequential Υ suppression in Au+Au collisions at √s_NN = 200 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) through both the dielectron and dimuon decay channels. In the 0-60% centrality class, the nuclear modification factors (R_AA), which quantify the level of yield suppression in heavy-ion collisions compared to pp collisions, for Υ(1S) and Y(2S) are 0.40 ± 0.03(stat.) ± 0.03(sys.) ± 0.09(norm.) and 0.26± 0.08(stat.) ± 0.02(sys.) ± 0.06(norm), respectively, while the upper limit of the Υ(3S) R_AAis 0.17 at a 95% confidence level. This provides experimental evidence that the Υ(3S) is significantly more suppressed than the Υ(1S) at RHIC. The level of suppression for Υ(1S) is comparable to that observed at the much higher collision energy at the Large Hadron Collider. These results point to the creation of a medium at RHIC whose temperature is sufficiently high to strongly suppress excited Y states.

Figures:

Figure 1. Invariant mass distributions of Υ candidates for 0 < p_T < 10 GeV/c reconstruc ted via the di-muon channel (top) for | y | < 0.5 and di-electron (bottom) channel for | y | < 1. Unlike-sign (full circles) and like-sign (open circles) distributions are simultaneously fitted with the unbinned maximum likelihood method. The like-sign distribution is used to estimate combinatorial background. The shapes of residual background from bb and Drell-Yan process are determined by simulation.

Figure 2. Υ(1S) (circles), and Υ(2S) (squares) R_AA as a function of N_partfor p_T < 10 GeV/c. The vertical bars on data points indicate statistical errors, while the systematic uncertainties are indicated as boxes. Shadowed bands around each marker depict the systematic uncertainties from N_coll. The bands at unity on the right side shows the global uncertainties from the p+p reference for Υ(1S) and Υ(2S),respectively.

Figure 3. Υ(1S) (top) and Υ(2S) (bottom) R_AA as a function of N_part for p_T < 10 GeV/c, compared to measurements (open symbols) in Pb+Pb collisions at √s_NN = 5.02 TeV, as well as different model calculations. The two bands at unity indicate the global uncertainties with the left one for CMS and the right one for STAR.

Figure 4. Υ(1S) (top),Υ(2S) (bottom) R_AA as a function of p_T in the 0-60% centrality, compared to different model calculations. The boxes and brackets around the data points represent systematic uncertainties from Au+Au analysis and p+p reference, respectively. The band at unity shows the uncertainty in N_coll.

Summary:

In summary, we report on measurements of Υ mesons suppression in Au+Au collisions at √sNN = 200 GeV with the STAR detector at RHIC.

For the first time, Υ(1S) and Υ(2S) suppression are measured separately at RHIC in Au+Au collisions at √sNN = 200 GeV.

A sequential suppression for different Y states is observed, with Υ(1S) is significantly more suppressed than Υ(3S)

This is also the first measurement of pT dependence of Υ mesons suppression from STAR. No clear pT dependence of the suppression is observed.

Υ(1S) has a similar magnitude of suppression as observed in Pb+Pb collisions at √sNN = 5.02 TeV.

Different model calculations are consistent with the experiment measurements.

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MTD Upsilon paper

Measurement of sequential Upsilon suppression in Au+Au collisions at √sNN = 200 GeV with STAR experiment

PAs: Bingchu Huang, Te-chuan Huang, Xinjie Huang, Ziyang Li, Rongrong Ma, Lijuan Ruan, Zebo Tang, Pengfei Wang, Yi Wang, Zhangbu Xu, Chi Yang, Qian Yang, Shuai Yang, Yi Yang, Zaochen Ye, Zhenyu Ye, Wangmei Zha, Shenghui Zhang, Zhe-jia Zhang, Xianglei Zhu

Target Journal: Phys. Rev. Lett.

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Abstract:

Summary:

Measurement of sequential Upsilon suppression in Au+Au collisions at √s_NN = 200 GeV with STAR experiment