Elliptic flow of strange and multi-strange hadrons in isobar collisions at $\sqrt{s_{\mathrm {NN}}} = 200\mathrm{~GeV}$ at RHIC

TL;DR

This study measures the elliptic flow $v_{2}$ of strange and multi-strange hadrons in isobar collisions $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr at $\sqrt{s_{\mathrm{NN}}}=200$ GeV to probe partonic collectivity and nuclear deformation. Using the STAR detector with the $\eta$-sub-event-plane method, the authors extract $v_{2}$ as a function of $p_T$ and centrality for $K^{0}_{S}$, $\Lambda$, $\overline{\Lambda}$, $\phi$, $\Xi$, $\overline{\Xi}$, and $\Omega$ , and test constituent-quark scaling by plotting $v_{2}/n_q$ versus $KE_T/n_q$. They find NCQ scaling holds within about 20%, while the $\,\langle v_{2}\rangle$ ratios between Ru+Ru and Zr+Zr deviate by ~2% in central/mid-central collisions, signaling differences in deformation and surface diffuseness; system-size trends show $v_{2}$ increasing with larger systems and AMPT-SM with deformed densities qualitatively reproducing the data. The results support partonic collectivity and quark coalescence as key mechanisms even in isobar collisions and provide constraints on nuclear structure models of Ru and Zr. Overall, this work enhances understanding of QGP-like collectivity in small-to-mid systems and informs nuclear-structure inputs used in dynamical models.

Abstract

We report a systematic measurement of elliptic flow ($v_{2}$) of $K_{s}^{0}$, $Λ$, $\overlineΛ$, $φ$, $Ξ^{-}$, $\overlineΞ^{+}$, and $Ω^{-}$+$\overlineΩ^{+}$ at mid-rapidity ($|y| < 1.0$) for isobar, $^{96}_{44}$Ru+$^{96}_{44}$Ru and $^{96}_{40}$Zr+$^{96}_{40}$Zr, collisions at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV. The transverse momentum ($p_\mathrm{T}$) dependence of $v_{2}$ is studied for various centrality classes. The number of constituent quark scaling of (multi-)strange hadrons is found to hold approximately within 20%, suggesting the development of partonic collectivity in isobar collisions similar to that observed in Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV. The average $v_{2}$ ratio shows $\sim$2% deviation from unity in central and mid-central collisions for strange hadrons, indicating a difference in nuclear structure and deformation between the isobars. The $v_{2}$ in isobaric collisions is compared to Cu+Cu, Au+Au, and U+U collisions at similar collision energies. We observe an increase in $v_{2}(p_\mathrm{T})$ with increasing system size. The difference in $v_{2}$ between the isobar and other collision systems increases with $p_\mathrm{T}$. The results are compared with a multi-phase transport model calculations with a deformed density profile to provide further insight into the nuclear structure of these isobars.

Figures (14)

• Figure 1: (a) The $\left\langle dE/dx \right\rangle$ distribution of particles identified using the TPC as a function of momentum (p) within $|\eta| <$ 1.0 for (a) Ru+Ru and (c) Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. The dashed lines represent the theoretical $\left\langle dE/dx \right\rangle$ values for the corresponding particle. Bottom panels: mass square as a function of p using TOF information in (b) Ru+Ru and (d) Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. The dashed lines represent the mass square values from PDG for the corresponding particle.
• Figure 2: Invariant mass distributions for (a) $K^{0}_{s}$, (b) $\phi$, (c) $\Lambda$, (d) $\overline{\Lambda}$, (e) $\Xi^{-}$, (f) $\overline{\Xi}^{+}$, and (g) $\Omega^{-}$+$\overline{\Omega}^{+}$ in minimum bias Ru+Ru collisions at $\sqrt{s_{NN}}$ = 200 GeV. The gray bands represent the combinatorial background. The rotational background technique is used for $K^{0}_{s}$, $\Lambda$, $\Xi$, and $\Omega$, while the mixed event technique is used for $\phi$ mesons. Error bars represent the statistical uncertainties.
• Figure 3: Invariant mass distributions for (a) $K^{0}_{s}$, (b) $\phi$, (c) $\Lambda$, (d) $\overline{\Lambda}$, (e) $\Xi^{-}$, (f) $\overline{\Xi}^{+}$, and (g) $\Omega^{-}$+$\overline{\Omega}^{+}$ in minimum bias Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. The gray bands represent the combinatorial background. Rotational background technique is used for $K^{0}_{s}$, $\Lambda$, $\Xi$, and $\Omega$, while mixed event technique is used for $\phi$ mesons. Error bars represent the statistical uncertainties.
• Figure 4: Second-order $\eta$-sub event plane angle ($\psi_{2}$) resolution obtained from the TPC as a function of centrality in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. The event plane resolution is compared with Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV lflowSTARUU.
• Figure 5: Raw yield as a function of $\phi-\psi_{2}$ for (a) $K^{0}_{s}$, (b) $\phi$, (c) $\Lambda$, (d) $\overline{\Lambda}$, (e) $\Xi^{-}$, (f) $\overline{\Xi}^{+}$, and (g) $\Omega^{-}$+$\overline{\Omega}^{+}$ at mid-rapidity ($|y| <$ 1.0) in minimum bias Ru+Ru collisions at $\sqrt{s_{NN}}$ = 200 GeV. The solid lines represent fits to the data to extract $v_{2}$. Error bars represent the statistical uncertainties.