The electromagnetic field of a fast charged particle, described as a flux of quasi-real photons whose intensity is proportional to its squared electric charge, provides copious photonuclear interactions in the case of lead ions circulating in the LHC. If the impact parameter of the colliding ions is larger than the sum of their radii, photon-induced processes dominate the interaction rate via ultra-peripheral collisions (UPC).
The cross section of a $\rho^0$ photonuclear production in Pb-Pb UPC at the LHC is so large that it becomes a proper tool to research the approach to the black-disk limit of QCD. ALICE presents the coherent $\rho^0$ photoproduction measurements. The results are compared with model predictions for different nuclear-breakup classes.
ALICE measured the J/$\psi$ photoproduction cross sections in Pb-Pb UPC at the forward and central rapidities. The results are compared to the newest models describing possible saturation and gluon shadowing at small $x$.