The coherent J/$ψ$ 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 ${\rm d} σ/ {\rm d}y$ is reported for the centrality range 40-90%, together with the doubly-differential cross section ${\rm d}^2 σ/{\rm d}y {\rm d} p_{\rm T}$, extracted in two peripheral centrality classes. The J/$ψ$ mesons are reconstructed in the dielectron channel, in the rapidity interval $|y| <~ 0.9$ using the ALICE central barrel detectors. The J/$ψ$ 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.
Phys. Lett. B 871 (2025) 139952
HEP Data
e-Print: arXiv:2409.11940 | PDF | inSPIRE
CERN-EP-2024-232
Figure group
Figure 1
Invariant mass distribution for \pt $$ 200 \MeVc (left panel) and \pt distribution for $2.92 \mee 3.16$ \GeVmass (right panel) in the centrality class 70--90\% for dielectrons with \mbox{$|y| 0.9$} in \PbPb collisions at \fivenn. Measured distributions for \jpsi candidates are shown as black markers and the fit components are shown as coloured lines, as explained in the legend. |  |
Figure 2
Left: Coherent \jpsi cross section as a function of $\langle \Npart \rangle$ in \PbPb collisions at \fivenn 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 {\em et al.} , Gay-Ducati {\em et al.} (GBW S2/S3 and IIM S2/S3) , and Cepila {\em et al.} (GG-hs) . For the interval 70--90\%, the calculations from W. Zha {\em et al.} and IIM S2 are shifted on the x-axis for visibility. Right: Coherent \jpsi 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 \jpsi cross section as a function of \pt in \PbPb collisions at \fivenn 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 {\em et al.} . |   |