Overview of recent UPC measurements

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/$ψ$ 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 $ρ^{0}$ photoproduction exhibits impact-parameter-dependent azimuthal anisotropy consistent with quantum interference effects, while the first polarisation measurement of coherently photoproduced J/$ψ$ 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.

Figures (6)

• Figure 1: (a) Energy distributions for single-proton emission accompanied by neutrons measured in ZNC (left) and ZNA (right) in EMD events. (b) Isotope production (Pb, Tl, Hg, Au) in nuclear breakup in Pb--Pb UPCs protonemission.
• Figure 2: Energy dependence of the incoherent J/$\psi$ photonuclear cross section off Pb for three $|t|$ intervals, compared with shadowing- and saturation-based model predictions gluonsaturation.
• Figure 3: Asymmetric pseudorapidity distributions resulting from asymmetric vetoing of forward detector activity in UPC event selection.
• Figure 4: (a) Inclusive D$^{0}$ production as a function of $p_{T}$ in the midrapidity region $|y|< 0.9$, compared with CMS results and theoretical model calculations opencharm. (b) Invariant-mass distribution of reconstructed $\Lambda_{\rm c}^{+}$ candidates.
• Figure 5: (a) Yield ratios K/$\pi$ and p/$\pi$ in inclusive $\gamma$--Pb UPCs compared to STARlight+DPMJET predictions. (b) Ratio of $\Lambda$ and $\bar{\Lambda}$ to K$^{0}_{\rm S}$ in Pb--Pb UPCs and p--Pb collisions as a function of $p_{\rm T}$.