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 $ψ$(2S) to J/$ψ$ as a function of transverse momentum is reported, along with the centrality dependence of the double ratio, defined as the $ψ$(2S) to J/$ψ$ 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 $ψ$(2S) is significantly more suppressed than J/$ψ$ 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.

Figures (4)

• Figure 1: Signal significance of $\psi$(2S) and J/$\psi$ as a function of BDT threshold in 0--80% Ru+Ru and Zr+Zr collisions. The solid markers represent significances extracted from data, while solid curves represent the expected significances from the machine learning. Error bars around data points and bands around solid curves indicate statistical uncertainties. The measured and expected distributions are aligned at the threshold value of 0.5, as indicated by the open boxes. The signal significance corresponding to the default BDT threshold is indicated by stars.
• Figure 2: Invariant mass distributions of $\psi$(2S) and J/$\psi$ candidates for $p_{\rm T} > 0.2$ GeV/$c$ and $|y| < 1$ in 0--80% Ru+Ru and Zr+Zr collisions. The combinatorial background estimated using the mixed-event technique is shown as triangles. Background-subtracted distributions (full circles) are fitted. Fit results are shown by the solid curves, while curves of other styles represent individual components included in the fit. The bands around the residual background fit curves display fitting errors. The mass ranges used for determining the signal counts are highlighted as vertical shaded bands.
• Figure 3: Double ratios of $\psi$(2S) to J/$\psi$ as a function of $\langle N_{\rm part}\rangle$NA50:2006yzzALICE:2022jeh are shown in the left panel. For the 0--20% centrality measurements, the upper limit with 95% confidence level is shown. The result integrated over centrality ($\langle N_{\rm part}\rangle$ = 63) for Ru+Ru and Zr+Zr collisions is presented in the right panel. For measurements from heavy-ion collisions, the error bars indicate statistical uncertainties, while the boxes denote systematic uncertainties. Other results PHENIX:2013pmn only display total uncertainties, sum of statistical and systematic uncertainties in quadrature. The bands around unity indicate the uncertainty in the reference $p$+$p$ value.
• Figure 4: Transverse momentum ($p_{\rm T}$) dependence of the $\psi$(2S) to J/$\psi$ yield ratio in $p$+$p$PHENIX:2011gybSTAR:2019vktArsene:2024fdb and heavy-ion collisions. The vertical bars and boxes around data points represent statistical and systematic uncertainties, respectively, while the horizontal bars indicate bin widths.