$\mathrm{{\rm Y}}$ production in U + U collisions at $\sqrt{s_{NN}}=193$ GeV measured with the STAR experiment

We present a measurement of the inclusive production of $\mathrm{{\rm Y}}$ mesons in U+U collisions at $\sqrt{s_{NN}}=193$ GeV at midrapidity $\left(\left|y\right|<1\right)$. Previous studies in central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV show a suppression of $\mathrm{{\rm Y}}\left(\text{1S+2S+3S}\right)$ production relative to expectations from the $\mathrm{{\rm Y}}$ yield in p+p collisions scaled by the number of binary nucleon-nucleon collisions $\left(N_{\mathrm{coll}}\right)$, with an indication that the $\mathrm{{\rm Y}}\left(\text{1S}\right)$ state is also suppressed. The present measurement extends the number of participant nucleons in the collision ($N_{\mathrm{part}}$) by 20% compared to Au+Au collisions, and allows us to study a system with higher energy density. We observe a suppression in both the $\mathrm{{\rm Y}}\left(1\mathrm{S}+2\mathrm{S}+3\mathrm{S}\right)$ and $\mathrm{{\rm Y}}\left(1\mathrm{S}\right)$ yields in central U+U data, which consolidates and extends the previously observed suppression trend in Au+Au collisions.

Figure 1

Single electron efficiency of the $dE/dx$ cut versus transverse momentum, as determined by fits to $n{\sigma }_e$ distributions of photonic electrons. The fit errors using the sample with the $m_{ee}<150$ MeV/$c^2$ photonic electron cut in 1 GeV/$c$ wide bins are used as systematic uncertainties. The results using the $m_{ee}<50$ MeV/$c^2$ photonic electron cut are consistent with the former one.

Figure 2

Single electron efficiency of the $E_{\mathrm{cluster}}/p$ cut versus transverse momentum. The efficiency corrections are obtained from embedded simulations. The difference between the default result from simulations and that extracted using a pure electron sample from data is taken as the systematic uncertainty.

Figure 3

Reconstruction efficiencies for $\mathrm{{\rm Y}}\left(\text{1S}\right), \mathrm{{\rm Y}}\left(\text{2S}\right)$, and $\mathrm{{\rm Y}}\left(\text{3S}\right)$, as determined from embedded simulations and identified electron samples. Cuts for (i) acceptance, triggering, and tracking, (ii) specific energy loss, (iii) track-cluster matching, (iv) $E_{\mathrm{cluster}}/p$, and (v) cluster compactness ($E_{\mathrm{tower}}/E_{\mathrm{cluster}}$) are applied consecutively to build up the total reconstruction efficiency. The efficiencies corresponding to each cut are shown stacked in a top-to-bottom order. Black ticks at the end of each bar represent the total uncertainties on the given efficiency. The $p_{\mathrm{T}}$ -binned values correspond to 0%–60% centrality.

Figure 4

Reconstructed invariant mass distribution of $\mathrm{{\rm Y}}$ candidates (unlike-sign pairs, denoted as solid circles) and like-sign combinatorial background (open circles) in U+U collisions at $\sqrt{s_{NN}}=193$ GeV for 0%–60% centrality at midrapidity ($\left|y\right|<1$). Fits to the combinatorial background, $b\overline{b}$ and Drell-Yan contributions, and to the $\mathrm{{\rm Y}}$ peaks are plotted as dash-dotted, dashed, and solid lines, respectively. The fitted contributions of the individual $\mathrm{{\rm Y}}\left(\text{1S}\right), \mathrm{{\rm Y}}\left(\text{2S}\right)$, and $\mathrm{{\rm Y}}\left(\text{3S}\right)$ states are shown as dotted lines.

Figure 5

Reconstructed invariant mass distribution of $\mathrm{{\rm Y}}$ candidates (solid circles) and like-sign combinatorial background (open circles) in U+U collisions at $\sqrt{s_{NN}}=193$ GeV for $p_{\mathrm{T}}$-integrated 0%–10% (a), 10%–30% (b), 30%–60% (c) centralities at midrapidity ($\left|y\right|<1$). Fits to the combinatorial background, $b\overline{b}$ and Drell-Yan contributions, and the peak fits are plotted as dash-dotted, dashed, and solid lines, respectively.

Figure 6

$\mathrm{{\rm Y}}\left(1\mathrm{S}+2\mathrm{S}+3\mathrm{S}\right)$ (a) and $\mathrm{{\rm Y}}\left(\text{1S}\right)$ (b) $R_{\mathrm{AA}}$ vs $N_{\mathrm{part}}$ in $\sqrt{s_{NN}}=193$ GeV U+U collisions (solid circles), compared to 200 GeV RHIC Au+Au (solid squares [13] and hollow crosses [32]), and 2.76 TeV LHC Pb+Pb data (solid diamonds [33]). A 95% lower confidence bound is indicated for the 30%–60% centrality U+U data (see text). Each point is plotted at the center of its bin. Centrality integrated (0%–60%) U+U and Au+Au data are also shown as open circles and squares, respectively.

Figure 7

$\mathrm{{\rm Y}}\left(1\mathrm{S}+2\mathrm{S}+3\mathrm{S}\right)$ (a) and $\mathrm{{\rm Y}}\left(\text{1S}\right)$ (b) $R_{\mathrm{AA}}$ vs $N_{\mathrm{part}}$ in $\sqrt{s_{NN}}=193$ GeV U+U collisions (solid circles), compared to different models [36, 37, 38], described in the text. The 95% lower confidence bound is indicated for the 30%–60% centrality U+U data (see text). Each point is plotted at the center of its bin. Centrality integrated (0%–60%) U+U and Au+Au data are also shown as open circles and squares, respectively.

Figure 8

Quarkonium $R_{\mathrm{AA}}$ versus binding energy in Au+Au and U+U collisions. Open symbols represent 0%–60% centrality data; filled symbols are for 0%–10% centrality. The $\mathrm{{\rm Y}}$ measurements in U+U collisions are denoted by red points. In the case of Au+Au collisions, the $\mathrm{{\rm Y}}\left(\text{1S}\right)$ measurement is denoted by a blue square, while for the $\mathrm{{\rm Y}}\left(2\mathrm{S}+3\mathrm{S}\right)$ states, a blue horizontal line indicates a 95% upper confidence bound. The black diamonds mark the high-$p_{\mathrm{T}} J/\psi$ measurement. The vertical lines represent nominal binding energies for the $\mathrm{{\rm Y}}\left(\text{1S}\right)$ and $J/\psi$, calculated based on the mass defect, as $2m_D-m_{J/\psi }$ and $2m_B-m_{\mathrm{{\rm Y}}}$, respectively (where $m_X$ is the mass of the given meson $X$) [39]. The shaded area spans between the binding energies of $\mathrm{{\rm Y}}\left(\text{2S}\right)$ and $\mathrm{{\rm Y}}\left(\text{3S}\right)$. The data points are slightly shifted to the left and right from the nominal binding energy values to improve their visibility.