UPCs as probes of partonic structure -- exclusive and inclusive processes

TL;DR

This paper reviews how ultraperipheral collisions at high-energy colliders serve as powerful probes of the partonic structure of protons and nuclei, exploring phenomena like gluon shadowing and saturation through various photoproduction processes.

Contribution

It provides a comprehensive overview of phenomenological applications of collinear factorization and dipole models to UPCs, highlighting new insights into small-x QCD dynamics and nuclear parton distributions.

Findings

Strong evidence for gluon nuclear shadowing in Pb-Pb UPCs

Potential signs of gluon saturation onset in nuclei

Constraints on nuclear parton distributions from dijet photoproduction

Abstract

Ultraperipheral collisions (UPCs) at the LHC and RHIC provide important new information on the partonic structure of the proton and nuclei and small-$x$ dynamics in QCD. We review phenomenological applications of the collinear factorization at leading and next-to-leading orders of perturbative QCD and the dipole model to coherent and incoherent $J/\psi$ photoproduction in Pb-Pb UPCs at the LHC emphasizing the strong leading twist gluon nuclear shadowing, the role of quark-antiquark-gluon dipoles, and a possible onset the gluon saturation in nuclei. We also discuss inclusive and diffractive dijet photoproduction in UPCs, which give complementary constraints on nuclear parton distributions and the pattern of factorization breaking in diffraction.