Probing parton dynamics of QCD matter with $\mathrm{\Omega }$ and $\varphi$ production

We present measurements of $\mathrm{\Omega }$ and $\varphi$ production at midrapidity from Au+Au collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}=7.7, 11.5, 19.6$, 27, and 39 GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation mechanism for these strange hadrons, we study the ratios of $N({\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+})/\left[2N\left(\varphi \right)\right]$. These ratios as a function of transverse momentum $p_T$ fall on a consistent trend at high collision energies, but start to show deviations in peripheral collisions at $\sqrt{s_{NN}}=19.6$, 27, and 39 GeV, and in central collisions at 11.5 GeV in the intermediate $p_T$ region of $2.4-3.6$ GeV/$c$. We further evaluate empirically the strange quark $p_T$ distributions at hadronization by studying the $\mathrm{\Omega }/\varphi$ ratios scaled by the number of constituent quarks (NCQ). The NCQ-scaled $\mathrm{\Omega }/\varphi$ ratios show a suppression of strange quark production in central collisions at 11.5 GeV compared to $\sqrt{s_{NN}}\ge 19.6$ GeV. The shapes of the presumably thermal strange quark distributions in 0–60% most central collisions at 7.7 GeV show significant deviations from those in 0–10% most central collisions at higher energies. These features suggest that there is likely a change of the underlying strange quark dynamics in the transition from quark matter to hadronic matter at collision energies below 19.6 GeV.

Figure 1

Midrapidity ($\left|y\right|<0.5$) $\varphi$ meson $p_T$ spectra from Au+Au collisions at different centralities and energies ($\sqrt{s_{NN}}=7.7$–39 GeV). The red boxes represent systematic errors. The dashed curves represent fits to the experimental data with a Levy function [9].

Figure 2

Midrapidity ($\left|y\right|<0.5$) ${\mathrm{\Omega }}^{-}$(${\overline{\mathrm{\Omega }}}^{+}$) $p_T$ spectra from Au+Au collisions at different centralities and energies ($\sqrt{s_{NN}}=7.7$–39 GeV). The open symbols represent ${\overline{\mathrm{\Omega }}}^{+}$ and solid symbols represent ${\mathrm{\Omega }}^{-}$. The red boxes denote systematic errors. The dashed curves represent fits to the experimental data with a Levy function [9].

Figure 3

Baryon-to-meson ratio, $N({\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+})/\left[2N\left(\varphi \right)\right]$, as a function of $p_T$ in midrapidity ($\left|y\right|<0.5$) from Au+Au collisions at $\sqrt{s_{NN}}=7.7$–200 GeV. Error bars and boxes represent statistical and systematic uncertainties, respectively. $\mathrm{\Omega }$ and $\varphi$ systematic errors are mostly from uncorrelated contributions. The solid and dashed lines represent recombination model calculations for central collisions at $\sqrt{s_{NN}}=200$ GeV [14] with total and thermal strange quark contributions, respectively.

Figure 4

Centrality dependence of $N({\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+})/\left[2N\left(\varphi \right)\right]$ ratios, as a function of $p_T$ in midrapidity ($\left|y\right|<0.5$) from Au+Au collisions at $\sqrt{s_{NN}}=11.5$, 19.6, 27, and 39 GeV. The boxes denote systematic errors.

Figure 5

(a) NCQ-scaled $N({\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+})/\left[2N\left(\varphi \right)\right]$ ratios, $k_s\left(p_T^s\right)$, as a function of $p_T\text{/}{n}_q$ in midrapidity ($\left|y\right|<0.5$) from Au+Au collisions at $\sqrt{s_{NN}}=7.7$–200 GeV. Here $n_q$ is the number of constituent quarks of each hadron. The boxes denote systematic errors. Dashed lines are Boltzmann fits to data. (b) The fitting parameters $A$ and $T$, and $1\sigma$ contours (including statistic and systematic errors).