The Physics of Ultraperipheral Collisions at the LHC

TL;DR

This paper explores ultraperipheral collisions at the LHC, highlighting their potential to probe small-x QCD phenomena, parton densities, and high gluon density regimes through photon-nucleon interactions and two-photon processes.

Contribution

It provides a comprehensive analysis of UPC physics at the LHC, detailing how current detectors can extend the exploration of small-x phenomena and nonlinear QCD effects beyond previous experiments.

Findings

LHC UPCs can explore photon-nucleon interactions at energies above 1 TeV.

Probing diffractive and inclusive parton densities in nuclei is feasible.

UPC processes can reveal the onset of high gluon density QCD regimes.

Abstract

We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon-nucleon (nucleus) interactions. The current LHC detector configurations can explore small $x$ hard phenomena with nuclei and nucleons at photon-nucleon center-of-mass energies above 1 TeV, extending the $x$ range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic $J/\psi$ and $\Upsilon$ production as well as in high $t$ $\rho^0$ production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.