The production of the $\psi(2S)$ charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ 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 J/$\psi$ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_{\rm T}<~12$ GeV/$c$. The results are compared with the corresponding measurements in pp collisions, by forming the double ratio $[\sigma^{\psi(2S)}/\sigma^{J/\psi}]_{\rm{Pb-Pb}}/[\sigma^{\psi(2S)}/\sigma^{J/\psi}]_{\rm{pp}}$. 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 $R_{\rm AA}$ was also obtained as a function of both centrality and $p_{\rm T}$. The results show that the $\psi(2S)$ resonance yield is strongly suppressed in Pb-Pb collisions, by a factor up to $\sim 3$ with respect to pp. Comparisons of cross section ratios with previous SPS findings by the NA50 experiment and of $R_{\rm AA}$ with higher-$p_{\rm T}$ 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.
Phys. Rev. Lett. 132 (2024) 042301
HEP Data
e-Print: arXiv:2210.08893 | PDF | inSPIRE
CERN-EP-2022-210
Figure group
Figure 1
Ratio of the $\psi({\rm 2S})$ and J/$\psi$ inclusive cross sections, not corrected for the corresponding branching ratios (BR) of the dimuon decay, as a function of $\langle N_{\rm part}\rangle$. The vertical error bars and the filled boxes represent statistical and systematic uncertainties, respectively. Data are compared to predictions of the TAMU and SHMc models, the corresponding lines showing their uncertainty band, and to results of the SPS NA50 experiment . 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 A.1
The opposite-sign invariant mass spectrum of mass pairs, for the four centrality intervals used in the data analysis, after mixed-event background subtraction. The kinematic domain is $2.5< y< 4$, $p_{\rm T}< 12$ GeV/$c$ ($0.3< p_{\rm T}< 12$ GeV/$c$ for the two most peripheral intervals). The result of one among the 60 $\chi^2$ minimization fit is reported, corresponding to a CB function for the signal, a variable-width Gaussian for the background, $2< m_{\mu\mu}< 5$ GeV/$c^2$ fitting range, a $2< m_{\mu\mu}< 8$ GeV/$c^2$ normalization range of the mixed-event background, and CB tails obtained from Monte Carlo. The contribution of the J/$\psi$ and $\psi(2S)$ resonances and of the background continuum as obtained from the fit are also reported. |     |
Figure A.2
The opposite-sign invariant mass spectrum of mass pairs, for the four transverse momentum intervals used in the data analysis, after mixed-event background subtraction. The kinematic domain is $2.5< y< 4$ and the centrality interval is 0--90\%. The result of one among the 60 $\chi^2$ minimization fit is reported, corresponding to a CB function for the signal, a variable-width Gaussian for the background, a $2< m_{\mu\mu}< 5$ GeV/$c^2$ fitting range, $2< m_{\mu\mu}< 8$ GeV/$c^2$ normalization range of the mixed-event background, and CB tails obtained from Monte Carlo. The contribution of the J/$\psi$ and $\psi(2S)$ resonances and of the background continuum as obtained from the fit are also reported. |     |
Figure B.1
The ratio of the $\psi({\rm 2S})$ and J/$\psi$ cross sections as a function of $p_{\rm T}$, not corrected for the corresponding branching ratios BR of the dimuon decay, compared to measurements in pp collisions . In the lower panel the double ratios of the Pb--Pb and pp values are shown. |  |