Observation of charmonium sequential suppression in heavy-ion collisions at the Relativistic Heavy Ion Collider

Abstract

We report measurements of charmonium sequential suppression in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The inclusive yield ratio of $\psi$(2S) to J/$\psi$ as a function of transverse momentum is reported, along with the centrality dependence of the double ratio, defined as the $\psi$(2S) to J/$\psi$ ratio in heavy-ion collisions relative to that in $p$+$p$ collisions. In the 0-80% centrality class, the double ratio is found to be 0.41 $\pm$ 0.10 (stat) $\pm$ 0.03 (syst) $\pm$ 0.02 (ref), lower than unity with a significance of 5.6 standard deviations. This provides experimental evidence that $\psi$(2S) is significantly more suppressed than J/$\psi$ in heavy-ion collisions at RHIC. This sequential suppression pattern seems to increase from peripheral to central collisions, but with no significant dependence on the transverse momentum.