Fast moving highly-charged ions carry strong electromagnetic fields that act as a beam of photons. In collisions at large impact parameters (b>R1+R2), hadronic interactions are not possible, and the ions interact through photon-ion and photon-photon collisions known as ultra-peripheral collisions (UPCs). Ultra-peripheral hadron-hadron collisions will provide unique opportunities for studying electromagnetic processes. Ultra-relativistic heavy-ion interactions have been used to study nuclear photo excitation (e.g., to a Giant Dipole Resonance) and photoproduction of hadrons.
Hadron colliders like the Relativistic Heavy Ion Collider (RHIC), the Tevatron, and the Large Hadron Collider (LHC) produce photonuclear and two-photon interactions at luminosities and energies beyond that accessible elsewhere.
