Photoproduction at the Relativistic Heavy Ion Collider with STAR

Abstract

The strong electromagnetic fields of short duration associated with relativistic heavy ions make a heavy-ion collider a unique tool to study two-photon and photonuclear collisions. In this talk, we introduce the principles of photoproduction at hadron colliders, review recent results from RHIC on meson and e$^+$e$^-$ production. At RHIC, STAR has studied exclusive $\rho^{0}$ vector meson production and $\rho^{0}$ production accompanied by electromagnetic dissociation of both nuclei in collisions of AuAu at 62, 130 and 200 GeV. Recent results suggest the validity of the Glauber calculations for the vector meson photoproduction and incosistency of the model based on the parton saturation phenomenon. The measurements are a lso sensitive to interference between production on the two nuclei: either ion can be the photon emitter or the target. The level of observed interference suggests that the final state wave function carries information about all possible decays long after the decay occurs. We also observe coherent photoproduction of a $\pi ^{+}\pi^{-}\pi^{+}\pi^{-}$ state which may be associated with $\rho^{0*}$ (1450).