# Ultra-peripheral collisions and hadronic structure

## TL;DR

Ultra-peripheral collisions at the LHC provide unprecedented access to gluon distributions in protons and heavy ions at very low Bjorken-x, enabling new insights into hadronic structure through photon-mediated interactions.

## Contribution

This review highlights recent experimental measurements of dilepton production, light-by-light scattering, and hadronic structure in ultra-peripheral collisions, emphasizing data from Quark Matter 2017.

## Key findings

- Probing gluon distributions at x down to 3×10^{-6} in protons.
- High-statistics ρ^0 data reveal spatial distribution of interaction sites.
- Recent measurements improve understanding of hadronic structure at low Bjorken-x.

## Abstract

Ultra-peripheral collisions are the energy frontier for photon-mediated interactions, reaching, at the Large Hadron Collider (LHC), $\gamma-p$ center of mass energies five to ten times higher than at HERA and reaching $\gamma\gamma$ energies higher than at LEP. Photoproduction of heavy quarkonium and dijets in $pp$ and $pA$ collisions probes the gluon distribution in protons at Bjorken-$x$ values down to $3\times10^{-6}$, far smaller than can be otherwise studied. In $AA$ collisions, these reactions probe the gluon distributions in heavy ions, down to $x$ values of a few $10^{-5}$. Although more theoretical work is needed to nail down all of the uncertainties, inclusion of these data in current parton distribution function fits would greatly improve the accuracy of the gluon distributions at low Bjorken-$x$ and low/moderate $Q^2$. High-statistics $\rho^0$ data probe the spatial distribution of the interaction sites; the site distribution is given by the Fourier transform of $d\sigma/dt$. After introducing UPCs, this review presents recent measurements of dilepton production and light-by-light scattering and recent data on proton and heavy nuclei structure, emphasizing results presented at Quark Matter 2017 (QM2017).