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\textbf{\Large Onset of Partonic Collectivity in Heavy-Ion Collisions at RHIC
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\textbf{Li-Ke Liu}\\

\textbf{(for the STAR Collaboration)}\\
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Partonic collectivity is one of the key signatures for the formation of quark-gluon-plasma in high-energy nuclear collisions. 
Scaling with the number of constituent quarks (NCQ) of elliptic flow ($v_2$) has been observed at
top RHIC energy ({\rootsNN} = 200 GeV) and at the LHC ({\rootsNN} = 2.76 TeV), indicating the presence of partonic collectivity. 
Meanwhile, NCQ scaling is absent for identified particles at {\rootsNN} = 3 GeV Au+Au collisions.

In this talk, we present a systematic analysis of $v_2$ for light, strange, multi-strange, and $\phi$ mesons in Au+Au collisions 
at {\rootsNN} = 3.0, 3.2, 3.5, 3.9, 4.5, 7.7, 9.2, 11.5, 14.5, 17.3, and 19.6 GeV, using data from the STAR experiment at RHIC. 
While NCQ scaling is clearly violated at {\rootsNN} $\le$ 3.2 GeV, implying a hadronic interaction-dominated equation of state, the scaling is gradually restored as the energy increases up to the 4.5 GeV Au+Au collisions, indicating the onset of the partonic interactions in these collisions. 
The $v_2$ results of multi-strange hadrons and $\phi$ mesons, with new high statistics data, 
will be shown for collision energies {\rootsNN} $\ge$ 7.7 GeV.
In addition, the energy dependence of the $p_T$-integrated $v_2$ for different particle species will be compared with predictions from transport model calculations.
Finally, we will discuss the implications of these measurements on understanding of QCD phase structure.


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