Ultra-peripheral collisions and hadronic structure

Understand

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 $Q^2$.