Probing small $x$ parton densities in ultraperipheral $AA$ and $pA$ collisions at the LHC

Abstract

We calculate production rates for several hard processes in ultraperipheral proton-nucleus and nucleus-nucleus collisions at the LHC. The resulting high rates demonstrate that some key directions in small $x$ research proposed for HERA will be accessible at the LHC through these ultraperipheral processes. Indeed, these measurements can extend the HERA $x$ range by roughly a factor of 10 for similar virtualities. Nonlinear effects on the parton densities will thus be significantly more important in these collisions than at HERA.

Submitted to Phys. Rev. Lett., 4 pages, 5 figures