Measurement of sequential $\Upsilon$ suppression in Au+Au collisions at $\sqrt{s_{_\mathrm{NN}}}$ = 200 GeV with the STAR experiment

TL;DR

This paper measures the suppression of different $5$ bottomonium states in gold-gold collisions at RHIC, showing that excited states are more suppressed, indicating a hot, dense medium capable of dissociating these particles.

Contribution

First measurement of sequential $5$ suppression at RHIC energies using dielectron and dimuon channels, demonstrating significant suppression of excited states.

Findings

$5$(1S) suppression factor is about 0.40

$5$(2S) suppression factor is about 0.26

$5$(3S) suppression is greater, with an upper limit of 0.17

Abstract

We report on measurements of sequential $\Upsilon$ suppression in Au+Au collisions at $\sqrt{s_{_\mathrm{NN}}}$ = 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 ($R_{\mathrm{AA}}$), which quantify the level of yield suppression in heavy-ion collisions compared to $p$+$p$ collisions, for $\Upsilon$(1S) and $\Upsilon$(2S) are $0.40 \pm 0.03~\textrm{(stat.)} \pm 0.03~\textrm{(sys.)} \pm 0.09~\textrm{(norm.)}$ and $0.26 \pm 0.08~\textrm{(stat.)} \pm 0.02~\textrm{(sys.)} \pm 0.06~\textrm{(norm.)}$, respectively, while the upper limit of the $\Upsilon$(3S) $R_{\mathrm{AA}}$ is 0.17 at a 95% confidence level. This provides experimental evidence that the $\Upsilon$(3S) is significantly more suppressed than the $\Upsilon$(1S) at RHIC. The level of suppression for $\Upsilon$(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 $\Upsilon$ states.