Measurement of Sequential $\mathrm{\Upsilon }$ Suppression in $\mathrm{Au}+\mathrm{Au}$ Collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ with the STAR Experiment

We report on measurements of sequential $\mathrm{\Upsilon }$ suppression in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ 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 $p+p$ collisions, for $\mathrm{\Upsilon }(1S)$ and $\mathrm{\Upsilon }(2S)$ are $0.40\pm 0.03(\mathrm{stat})\pm 0.03(\mathrm{sys})\pm 0.09(\mathrm{norm})$ and $0.26\pm 0.08(\mathrm{stat})\pm 0.02(\mathrm{sys})\pm 0.06(\mathrm{norm})$, respectively, while the upper limit of the $\mathrm{\Upsilon }(3S)$ $R_{AA}$ is 0.17 at a 95% confidence level. This provides experimental evidence that the $\mathrm{\Upsilon }(3S)$ is significantly more suppressed than the $\mathrm{\Upsilon }(1S)$ at RHIC. The level of suppression for $\mathrm{\Upsilon }(1S)$ 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 $\mathrm{\Upsilon }$ states.

Figure 1

Invariant mass distributions of $\mathrm{\Upsilon }$ candidates for $0<p_T<10\mathrm{GeV}/c$ reconstructed via the dimuon decay channel within $|y|<0.5$ (top) and the dielectron decay channel within $|y|<1$ (bottom). Unlike-sign and like-sign distributions are shown as full and open circles, respectively. Solid lines are fits to the unlike-sign distributions, while lines of other styles represent individual components included in the fit. See more details in the text.

Figure 2

Left: $\mathrm{\Upsilon }(1S)$ (circles) and $\mathrm{\Upsilon }(2S)$ (squares) $R_{AA}$ as a function of $N_{\mathrm{part}}$ for $p_T<10\mathrm{GeV}/c$. Data points for $\mathrm{\Upsilon }(2S)$ are displaced horizontally for better visibility. The vertical bars on data points indicate statistical errors, while the systematic uncertainties are shown as boxes. Shadowed bands around each marker depict the systematic uncertainties from $N_{\mathrm{coll}}$. The bands at unity indicate the global uncertainties. Right: $R_{AA}$ for various $\mathrm{\Upsilon }$ states, including the 95% upper limit for $\mathrm{\Upsilon }(3S)$, in 0%–60% $\mathrm{Au}+\mathrm{Au}$ collisions.

Figure 3

$\mathrm{\Upsilon }(1S)$ (top) and $\mathrm{\Upsilon }(2S)$ (bottom) $R_{AA}$ as a function of $N_{\mathrm{part}}$ for $p_T<10\mathrm{GeV}/c$, compared to similar measurements in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ (open symbols), as well as model calculations (bands). The two bands at unity indicate the global uncertainties with the left one for CMS and the right one for STAR.

Figure 4

$\mathrm{\Upsilon }(1S)$ (top) and $\mathrm{\Upsilon }(2S)$ (bottom) $R_{AA}$ as a function of $p_T$ in the 0%–60% centrality class, compared to different model calculations. The boxes and brackets around the data points represent systematic uncertainties from $\mathrm{Au}+\mathrm{Au}$ analysis and $p+p$ reference, respectively. The band at unity shows the uncertainty in $N_{\mathrm{coll}}$.