The coherent J/$\psi$ photoproduction cross section is measured for the first time at midrapidity in peripheral to semicentral Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The centrality differential cross section ${\rm d} \sigma/ {\rm d}y$ is reported for the centrality range 40-90%, together with the doubly-differential cross section ${\rm d}^2 \sigma /{\rm d}y {\rm d} p_{\rm T}$, extracted in two peripheral centrality classes. The J/$\psi$ mesons are reconstructed in the dielectron channel, in the rapidity interval $|y| <~$ 0.9 using the ALICE central barrel detectors. The J/$\psi$ cross section at midrapidity is statistically compatible to the earlier ALICE measurement at forward rapidity and at the same centre-of-mass energy, and shows only a mild centrality dependence over the covered range. Several sets of theoretical calculations taking into account the hadronic overlap in the collisions but ignoring possible final-state effects from a hot expanding medium are found to give a fairly good description of the current measurements within uncertainties.
Submitted to: PLB
e-Print: arXiv:2409.11940 | PDF | inSPIRE
CERN-EP-2024-232
Figure group
Figure 1
Invariant mass distribution for $\pt$ 200 MeV/$c$ (left panel) and $\pt$ distribution for $2.92 < m_{\rm ee} < 3.16$ GeV/$c^2$ (right panel) in the centrality class 70-90% for dielectrons with $|y| < 0.9$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV. Measured distributions for J/$\psi$ candidates are shown as black markers and the fit components as coloured lines, as explained in the legend. |  |
Figure 2
Left: Coherent J/$\psi$ cross section as a function of $\langle \Npart \rangle$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV at midrapidity. The statistical uncertainties are shown as bars, while the systematic uncertainties (excluding the global contribution from the beam luminosity) are shown as boxes. It should be noted that the interval 40-50% is half as wide as the two more peripheral ones, 50-70%, and 70-90%. Data are compared to calculations from W. Zha et al., Gay-Ducati et al. (GBW S2/S3 and IIM S2/S3) and Cepila et al. (GG-hs). For the interval 70--90\%, the calculations from W. Zha et al. and IIM S2 are shifted on the x-axis for visibility. Right: Coherent J/$\psi$ cross section as a function of $\langle \Npart \rangle$ measured at midrapidity (black markers) and forward rapidity (red markers) normalised to the corresponding cross sections measured in the same rapidity ranges in UPC and corrected for the centrality interval width $\Delta C$. |   |
Figure 3
Coherent J/$\psi$ cross section as a function of $\pt$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV measured in the 50-70% and 70-90% centrality classes. The error bars indicate statistical uncertainties, while the systematic uncertainties, excluding those considered as global (see text for details), are shown as boxes. Data are compared to calculations from W. Zha et al. |   |