Overview of recent UPC measurements

TL;DR

This paper reviews recent photon-induced process measurements in ultra-peripheral collisions at the LHC, highlighting new insights into gluon structure, collision geometry, and particle production mechanisms from Run 2 and Run 3 data.

Contribution

It provides a comprehensive overview of recent experimental results from ALICE on UPCs, including novel measurements of vector meson photoproduction, polarization, and photonuclear interactions, advancing understanding of nuclear gluon distributions and collision dynamics.

Findings

Suppression of energy evolution at large |t| supports saturation models.

Impact-parameter-dependent azimuthal anisotropy observed in coherent ρ0 photoproduction.

First polarization measurement of coherently produced J/ψ confirms s-channel helicity conservation.

Abstract

This contribution presents an overview of recent measurements of photon-induced processes in ultra-peripheral collisions (UPCs) performed with the ALICE experiment at the LHC. Results from Run~2 include detailed studies of exclusive vector meson photoproduction in Pb-Pb collisions. Measurements of incoherent J/$\psi$ photoproduction as a function of energy and the Mandelstam-$|t|$ variable, combined with electromagnetic dissociation (EMD) classification, show a suppression of the energy evolution at large $|t|$, favouring saturation-based descriptions of the gluon structure. Complementary measurements of proton emission in Pb-Pb UPCs constrain nuclear breakup mechanisms and provide a handle on collision geometry through EMD tagging. Coherent $\rho^{0}$ photoproduction exhibits impact-parameter-dependent azimuthal anisotropy consistent with quantum interference effects, while the first polarisation measurement of coherently photoproduced J/$\psi$ confirms $s$-channel helicity conservation. Additional measurements of exclusive multi-hadron photoproduction, including four-pion and charged kaon pair final states, probe resonance contributions and light vector meson couplings to photons and nuclear targets. Run~3 data enable the study of inclusive photonuclear interactions. Measurements of inclusive open charm photoproduction constrain the gluon content of nuclei in the perturbative QCD regime. Studies of identified hadron production and baryon-to-meson ratios in photonuclear collisions provide new information on particle production mechanisms and possible collective effects in photon-induced systems. Photon-photon interactions are also studied through performance and sensitivity analyses for the measurement of the anomalous magnetic moment of the tau lepton, together with projections for future measurements with ALICE~3.