Onset of Constituent Quark Number Scaling in Heavy-Ion Collisions at RHIC

Abstract

Partonic collectivity is one of the necessary signatures for the formation of quark-gluon plasma in high-energy nuclear collisions. Number of constituent quarks (NCQ) scaling has been observed for hadron elliptic flow $v_2$ in top energy nuclear collisions at the Relativistic Heavy Ion Collider and the LHC, and this has been theoretically suggested as strong evidence for partonic collectivity. In this Letter, a systematic analysis of $v_2$ of $\ddot{I}{€}^{\pm }$, $K^{\pm }$, $K_S^0$, $p$, and $\hat{I}›$ in Au+Au collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}$ = 3.2, 3.5, 3.9, and 4.5 GeV, with the STAR experiment at the Relativistic Heavy Ion Collider, is presented. NCQ scaling is markedly violated at 3.2 GeV, consistent with a hadronic-interaction dominated equation of state. However, as the collision energy increases, a gradual evolution to NCQ scaling is observed. This beam-energy dependence of $v_2$ for all hadrons studied provides evidence for the onset of dominant partonic interactions by $\sqrt{s_{{}_{\mathrm{NN}}}}$ = 4.5 GeV.

8 pages, 4 figures