$\psi (2S)$ Suppression in Pb-Pb Collisions at the LHC

The production of the $\psi (2S)$ charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ($2.5<y<4$). The measurement of the ratio of the inclusive production cross sections of the $\psi (2S)$ and $\mathrm{J}/\psi$ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_T<12\mathrm{GeV}/c$. The results are compared with the corresponding measurements in $pp$ collisions, by forming the double ratio $[{\sigma }^{\psi (2S)}/{\sigma }^{J/\psi }{]}_{\mathrm{Pb}\text{−}\mathrm{Pb}}/[{\sigma }^{\psi (2S)}/{\sigma }^{J/\psi }{]}_{pp}$. It is found that in Pb-Pb collisions the $\psi (2S)$ is suppressed by a factor of $\sim 2$ with respect to the $J/\psi$. The $\psi (2S)$ nuclear modification factor $R_{\mathrm{AA}}$ was also obtained as a function of both centrality and $p_T$. The results show that the $\psi (2S)$ resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to $\sim 3$ with respect to $pp$. Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of $R_{\mathrm{AA}}$ with higher-$p_T$ results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC.

Figure 1

Ratio of the $\psi (2S)$ and $J/\psi$ inclusive cross sections, not corrected for the corresponding branching ratios (BR) of the dimuon decay, as a function of $⟨N_{\mathrm{part}}⟩$. The vertical error bars and the filled boxes represent statistical and systematic uncertainties, respectively. Data are compared to predictions of the TAMU [15] and SHMc [18, 47] models, the corresponding lines showing their uncertainty band, and to results of the SPS NA50 experiment [27]. In the lower panel, the ratios are normalized to the corresponding $pp$ value (double ratio). The filled boxes around the line at unity indicate the global systematic uncertainties.

Figure 2

The $R_{\mathrm{AA}}$ for the $\psi (2S)$ (this analysis) and $J/\psi$ [45] as a function of $⟨N_{\mathrm{part}}⟩$. Comparisons with models are also shown, the lines (band) showing the theory uncertainty for the SHMc (transport) calculations.

Figure 3

The $R_{\mathrm{AA}}$ for $\psi (2S)$ and $J/\psi$ [45] as a function of $p_T$. Comparison with theory models and results from the CMS experiment [31] are also shown.