This Paper presents rapidity-differential measurements of coherent J/$ψ$ and $ψ({\rm 2S})$ photoproduction, as well as rapidity- and mass-differential measurements of exclusive dimuon production, in the forward rapidity region $-4 <~ y <~ -2.5$ in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.36$ TeV using data recorded by the ALICE detector at the LHC in 2023, corresponding to an integrated luminosity of $\mathcal{L} = 1170 \pm 50~μ{\rm b}^{-1}$. The J/$ψ$ and $ψ({\rm 2S})$ results reveal the significant role of nuclear shadowing effects. The square root of the ratio of the measured quarkonium cross section to the impulse approximation prediction is about 0.76 for J/$ψ$ and 0.71 for $ψ({\rm 2S})$, at $y \approx -3$, corresponding to typical Bjorken-$x$ values of $10^{-2}$. The exclusive dimuon results highlight the sensitivity of such measurements to precise modeling of the photon flux, particularly at impact parameters near the nuclear radius.
Submitted to: JHEP
e-Print: arXiv:2605.13569 | PDF | inSPIRE
CERN-EP-2026-143
Figure group
Figure 2
Left: invariant mass distribution for the selected muon pairs with $\pt 0.25$ \GeVc in the full forward rapidity range. Right: transverse momentum distribution for muon pairs in the invariant mass range $2.8 m_{\mu\mu} 3.3$ \GeVmass. Fit components are described in the text. |   |
Figure 4
Measured rapidity-differential cross sections of the coherent \jpsi (left) and \psip (right) production in \PbPb UPCs at \fivethreesixnn. The results are compared with theoretical calculations described in the text. The statistical uncertainties are shown with bars, the systematic uncertainties are shown with boxes. |   |
Figure 5
Measured ratio of differential photoproduction cross sections of the coherent \jpsi and \psip in \PbPb UPCs at \fivethreesixnn. The results are compared with theoretical calculations described in the text. The statistical uncertainties are shown as bars, the systematic uncertainties are shown as boxes. |  |
Figure 6
Measured invariant-mass differential cross sections of the exclusive dimuon production $\gamma\gamma\to\mu\mu$ in \PbPb UPCs at \fivethreesixnn. The results are compared with predictions by Monte Carlo generators described in the text. The top part of the figures: the absolute values of the cross sections are presented. The bottom part of the figures: the ratios of data to Monte Carlo-based predictions are presented. The bars represent the total statistical and systematic uncertainty added in quadrature. |     |