The gluon distributions in the proton and nuclei at small x from photoproduction of charmonia in ultraperipheral collisions at the LHC
Vadim Guzey (Petersburg Nuclear Physics Institute, Gatchina)

Ultraperipheral collisions (UPCs) of ions at large impact parameters at hadron colliders allow one to study photon-proton and photon-nucleus interactions at high energies.
The focus of studies of pp, pA and AA UPCs at the Large Hadron Collider (LHC) has been photoproduction of charmonia (J/psi and psi(2S) mesons), which directly probes the gluon distributions in the proton and nuclei at small values of the momentum fraction x.
I will review the status of perturbative QCD (pQCD) analysis of the LHCb, ALICE and CMS UPC data and explain how the data constrain the small-x behavior of the proton gluon density down to x=6x10^{-6} and give evidence of the large leading-twist nuclear gluon shadowing at x=0.001. I will also discuss the outstanding problems with pQCD description of some of the data (incoherent J/psi and coherent psi(2S) photoproduction on nuclei) and possible future directions of UPC studies at the LHC during Run 2.