Speaker
Description
The cross section for coherent charmonia photonuclear production in ultra-peripheral collisions (UPCs) at the LHC is particularly sensitive to the low Bjorken-$x$ behaviour of the gluon distribution function of the interacting lead nuclei. It provides important constraints on the initial stages in heavy ion collisions. The new measurements of coherent $J/\psi$ and $\psi(2S)$ photoproduction by ALICE in Pb-Pb UPCs as well as the prospects of coherently produced $J/\psi$ mesons in UPCs of O-O, both at the forward rapidity using Run 3 data at $\sqrt{s_{\rm NN}} = 5.36$ TeV are reported. The measurements sample the gluon distribution of nucleus down to $x\sim 10^{-2}$. Results are compared to theoretical predictions based on gluon shadowing or saturation and indicate a large suppression w.r.t. the impulse approximation. The off-resonance exclusive dimuon cross section shows sensitivity to the photon flux modeling. The result shows different agreement of data measurement with various MC generator predictions.
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