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:

Partonic collectivity is one of the necessary signatures for the formation of quark-gluon-plasma in high-energy nuclear collisions. Number of constituents quark (NCQ) scaling have been observed for light hadrons $v_2$ at top energy nuclear collisions at RHIC and LHC presenting the partonic collectivity. 

In this letter, a systematic analysis of the elliptic flow $v_2$ of $\pi^{\pm}$, $K^{\pm}$, $K^{0}_{S}$, $p$ and $\Lambda$ in Au+Au collisions 

at √sNN = 3.2, 3.5, 3.9, and 4.5 GeV,  with the STAR experiment at RHIC, is represented. 

While the NCQ scaling is clearly violated at 3.2 GeV, implying a hadronic interactions dominated equation of state, the scaling is gradually restored when the energy is increased up to the 4.5 GeV Au+Au collisions demonstrating the onset of the partonic interactions in such collisions.

The energy dependence of the scaling property will be discussed within the framework of the hadronic transport model Jet AA Microscopic Transport Model (JAM) calculations.
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Paper figures:

Fig. 1. The transverse momentum ($p_T$) and identified particle rapidity (y) distribution for $\pi^{+}, K^{+}, K^{0}_{S}, p$ from Au+Au collisions at {\rootsNN} = 3.2, 3.5, 3.9, and 4.5 GeV. The blue boxes represent the acceptance ($-0.5 < y < 0 $) used for elliptic flow measurements.

Fig. 2. $p_T$ dependence of $v_2$ for $\pi^{\pm}, K^{\pm}, K^{0}_{S},p,\Lambda$ in 10-40\% centrality for Au+Au collisions at {\rootsNN} = 3.0, 3.2, 3.5, 3.9, and 4.5 GeV. Statistical and systematic uncertainties are shown as bars and bands, respectively. The JAM mean field results are shown as dashed bands: orange for 4.5 GeV, gray for 3.2 GeV.

Fig. 3. The number of constituent quark $n_q$ scaled $v_2$ as a function of $n_q$ scaled $E_{T}$ ($m_T - m_0$) for particles (upper panel) and anti-particles (lower panel) in 10-40\% centrality for Au+Au collisions at {\rootsNN} = 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. (a): The energy dependence of $p_T$ integrated $v_2$ for $\pi^{\pm}, K^{\pm}, K^{0}_{S},p,\Lambda$ in 10-40\% centrality from Au+Au collisions at {\rootsNN} = 3.0, 3.2, 3.5, 3.9, and 4.5 GeV. For clarity, the X-axis values of pions and kaons are shifted by $\pm 0.05$ respectively. 

(b): The energy dependence of NCQ scaled $v_{2}$ ratios for $\pi^{+} / K^{+}, p / K^{+}$ at $E_T/n_q$ = 0.4 GeV/$c^2$ in the same centrality and energies. Statistical and systematic uncertainties are shown as bars and bands, respectively. The JAM 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 present the elliptic flow of identified hadrons $\pi^{\pm}, K^{\pm}, K^{0}_{S}, p, \Lambda$ 

in Au+Au collisions at {\rootsNN} = 3.2, 3.5, 3.9, and 4.5 GeV. 

The $v_2$ of these particles changes from negative to positive around 3.2 GeV. At the lower colliding energy, {\rootsNN} $\leq$3.2 GeV, 

the calculations from the hadronic transport model JAM} reproduced the transverse momentum dependence of the measured $v_2(p_T)$ implying hadronic interaction dominant.

As collision energy increases, a gradual restoration of NCQ scaling is observed, and JAM calculations under-predict the collectivity observed in the data.

These observation indicate that partonic interactions become more important for collisions at  {\rootsNN} $\ge$ 3.5 GeV and the onset of the partonic collectivity.