Target journal: Phys. Rev. Lett.

PAs: Prabhupada Dixit, Xin Dong, Li-Ke Liu, Md. Nasim, Sooraj Radhakrishnan, Priyanshi Sinha, Shusu Shi, Guoping Wang, Xing Wu, Nu Xu, Shuai Zhou 

PA representative: Xing Wu
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Abstract:

Elliptic flow ($v_2$) is sensitive to the dynamics of heavy-ion collisions at the early stage of system evolution and the degrees of freedom of the medium. With the enhanced statistical datasets from the second phase of RHIC Beam Energy Scan (BES-II) program at STAR, measurements of $v_2$ for ${\pi}^{\pm}$, ${K}^{\pm}$, ${K}^{0}_{s}$, $p$, $\bar{p}$, ${\phi}$, ${\Lambda}$, $\bar{\Lambda}$, ${\Xi}^{\pm}$ and ${\Omega}^{\pm}$ at $\sqrt{s_{NN}}$ = 7.7 - 19.6 GeV are presented. Number of Constituent Quark (NCQ) scaling is tested separately for particles and anti-particles. The NCQ scaled $v_2$ ratios of ${\pi}^{+}$/${K}^{+}$, $p$/${K}^{+}$, ${\pi}^{-}$/${K}^{-}$, $\bar{p}$/${K}^{-}$, ${\phi}$/${K}^{-}$, ${\Lambda}$/${K}^{0}_{s}$, $\bar{\Lambda}$/${K}^{0}_{s}$ are found to be close to 1 and less sensitive to the collision energy within the range of $\sqrt{s_{NN}}$ = 7.7 - 19.6 GeV. These measurements indicate that the system is dominated by partonic degrees of freedom in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 - 19.6 GeV.

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Paper figures:

Fig.1.: Invariant mass distribution of $\phi$, $K^{0}_{S}$, $\Lambda$, $\bar{\Lambda}$, $\Xi^{\pm}$ and $\Omega^{\pm}$ at {\rootsNN} = 14.6 GeV.

Fig.2.: The NCQ-scaled elliptic flow, $v_2$/$n_q$ VS. ($m_T - m_0$)/$n_q$ for 10-40\% centrality in Au + Au collisions is shown for particles, along with the NCQ scaling ratios to the $K_S^0$ fitting function. The dash line represents the K$E_T/$$n_q$ range from 0.3 to 1.3. The bars and brackets represent the statistical and systematic errors respectively.

Fig.3.: The NCQ-scaled elliptic flow, $v_2$/$n_q$ VS. ($m_T - m_0$)/$n_q$ for 10-40\% centrality in Au + Au collisions is shown for anti-particles, along with the NCQ scaling ratios to the $K_S^0$ fitting function. The dash line represents the K$E_T/$$n_q$ range from 0.3 to 1.3. The bars and brackets represent the statistical and systematic errors respectively.

Fig.4.: (a): The energy dependence of NCQ-scaled $v_2$ ratios for $\pi^+$/$K^+$, $p$/$K^+$ and $\Lambda$/$K_S^0$ at K$E_T$/$n_q$ = 0.4 GeV/$c^2$ in 10-40\% centrality in Au + Au collisions at $\sqrt{s_{NN}}$ = 3.2 - 19.6 GeV. (b): The energy dependence of NCQ-scaled $v_2$ ratios for $\pi^-$/$K^-$, $\bar{p}$/$K^-$, $\phi$/$K^-$ and $\bar{\Lambda}$/$K_S^0$ at K$E_T$/$n_q$ = 0.4 GeV/$c^2$ in 10-40\% centrality in Au + Au collisions at $\sqrt{s_{NN}}$ = 7.7 - 19.6 GeV.