Paper Title: Onset of Partonic Collectivity in Heavy-Ion Collisions at RHIC

Target journal: Phys. Rev. Lett.

PAs: Xin Dong, Li-Ke Liu, Zuowen Liu, Sooraj Radhakrishnan, Shusu Shi, Guoping Wang, Xing Wu, Nu Xu
PA representative: Li-Ke Liu

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Abstract:

Elliptic flow (v_{2}) is sensitive to constituent interactions and degree of freedom of created matter in heavy-ion collisions. Number of Constituent Quark (NCQ) scaling on v_{2} at RHIC 200 GeV and LHC energy indicates that partonic collectivity has been built up, as a signal of the QGP is formed. With the energy decreasing, the NCQ scaling serves as a probe of the QCD phase transition, as the lack of partonic interaction distorts the scaling.
In this letter, we report the v_{2} measurements for $\pi^{\pm}, K^{\pm}, K^{0}_{S},p,\Lambda$ in Au+Au collisions at √sNN = 3.2, 3.5, 3.9, and 4.5 GeV.  The NCQ scaling is clearly violated at 3.2 GeV, indicating the hadronic dominant equation of state. As the collision energy increases, the scaling is gradually restored at the 4.5 GeV AuAu collisions implying that the partonic interactions become more important.

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Paper figures:

Fig. 1. The transverse momentum (p_{T}) and identified particle rapidity (y) distribution for π⁺, K⁺, K⁰_S, p from Au+Au collisions at √s_NN = 3.2, 3.5, 3.9, and 4.5 GeV.

Fig. 2. pT dependence of $\pi^{\pm}, K^{\pm}, K^{0}_{S},p,\Lambda$ in 10-40% centrality for Au+Au collisions at √s_NN = 3.0, 3.2, 3.5, 3.9, and 4.5 GeV. Statistical and systematic uncertainties are shown as bars and bands, respectively. The JAM2 mean field results are shown as dash bands: golden for 4.5 GeV, red for 3.2 GeV.

Fig. 3. Number of constituent quark scaling test of v_{2} for particles (upper panel), and anti-particles (lower panel) in 10-40% centrality for Au+Au collisions at √s_NN = 3.0, 3.2, 3.5, 3.9, and 4.5 GeV. Statistical and systematic uncertainties are shown as bars and bands, respectively.

Fig. 4. Upper: The energy dependence of pT integrated v_{2} for $\pi^{\pm}, K^{\pm}, K^{0}_{S},p,\Lambda$ in 10-40% centrality from Au+Au collisions at √s_NN = 3.0, 3.2, 3.5, 3.9, and 4.5 GeV. Lower: The energy dependence of NCQ ratio for $\pi^{+} / K^{+}$, $p / K^{+}$ in same centrality and energies. Statistical and systematic uncertainties are shown as bars and bands, respectively. The JAM2 mean field with spectator effect calculation are shown as color bands: grey for $\pi^{+} / K^{+}$, red for $p / K^{+}$.
 

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Conclusions:
In summary, we presented elliptic flow ($v_2$) measurements for $\pi^{\pm}$, $K^{\pm}$, $K_S^0$, p, and $\Lambda$ in Au + Au collisions at $\sqrt{s_{NN}}$ = 3.2, 3.5, 3.9, and 4.5 GeV. Particle identified $v_2$ increase as a function of collision energy and crossing zero at about 3.2 GeV,
it indicates shadowing effect due to the spectators play an important role within these energy region.
 The NCQ scaling is clearly violated at 3.2 GeV, indicating the hadronic dominant equation of state. As the collision energy increases, the scaling is gradually restored at the 4.5 GeV AuAu collisions implying that the partonic interactions become more important. Transport model calculations from both JAM and SMASH do not reproduce these experimental observations