  This paper deals with so-called Ultra-Peripheral Collisions (UPCs) of heavy
ions. These can be defined as collisions in which no hadronic interactions
occur because of the large spatial separation between the projectile and
target. The interactions are instead mediated by the electromagnetic field. Two
types of ultra-peripheral collisions can be distinguished: purely
electro-magnetic interactions (two-photon interactions) and photonuclear
interactions, in which a photon from the projectile interacts with the hadronic
component of the target.
