Peter Steinberg (Brookhaven)
Heavy ion collisions at RHIC and the LHC are perhaps best known for events in which the nuclei collide head-on, creating a volume of hot and dense matter which then decays into very high multiplicity final states. However, large (high Z) nuclei are also intense sources of high energy photons, which can interact with the other nucleus either directly or diffractively, or with each other in a gamma-gamma collision. These events are generally referred to as ultraperipheral collisions (UPC). This talk reviews results where pairs of oppositely charged leptons are produced either diffractively or electromagnetically, each of which probe different stages of the temporal evolution of the nuclear collision. Exclusive non-resonant dilepton events arise from gamma-gamma collisions and are sensitive to the very initial photon luminosity, an important input into theoretical calculations. Exclusive production of vector mesons results from photon-pomeron interactions and can thus probe the partonic structure of the nucleus. Finally, there are also classes of events which appear to be the simultaneous occurrence of a head-on heavy ion collision as well as a dilepton pair from electromagnetic processes. The systematic modification of the lepton pair kinematics suggests that they are potentially sensitive to strong magnetic fields, or even the microscopic properties of the hot, dense QGP.
Peter Steinberg (Brookhaven National Lab)