DIRECT PHOTON PRODUCTION IN RELATIVISTIC HEAVY-ION COLLISIONS

We examine the uses of direct photons in diagnosing the highly excited state of nuclear matter created in high-energy nuclear collisions. The traditional focus has been on direct photons as thermal radiation from the excited state, but we also explore the many other roles direct photons can play. We review experimental and theoretical techniques as well as the history of direct photon measurements in heavy-ion collisions and their interpretation.