Exclusive dimuon production and coherent charmonium photoproduction at forward rapidity in ultra-peripheral Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV

TL;DR

This paper reports measurements of coherent J/ψ, ψ(2S), and exclusive dimuon production in ultra-peripheral Pb-Pb collisions at 5.36 TeV, revealing nuclear shadowing effects and sensitivities to photon flux modeling.

Contribution

First measurements of these processes at this energy and rapidity, providing new insights into nuclear shadowing and photon flux modeling in ultra-peripheral collisions.

Findings

Nuclear shadowing reduces quarkonium cross sections to about 76% and 71% of impulse approximation.

Results are sensitive to photon flux modeling at impact parameters near the nuclear radius.

Data at Bjorken-x around 10^{-2} inform nuclear parton distribution functions.

Abstract

This Paper presents rapidity-differential measurements of coherent J/$\psi$ and $\psi({\rm 2S})$ 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 $\mathcal{L} = 1170 \pm 50~\mu{\rm b}^{-1}$. The J/$\psi$ and $\psi({\rm 2S})$ 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/$\psi$ and 0.71 for $\psi({\rm 2S})$, 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.