Photoproduction in Ultra-Peripheral Heavy Ion Collisions at STAR

Abstract

We present recent STAR results on photoproduction in ultra-peripheral relativistic heavy-ion collisions. In these collisions the impact parameter of the beam particles is larger than the sum of their nuclear radii, so that they interact via their long-range Coulomb fields. STAR has measured the production of ρ 0 ( 770 ) mesons in exclusive reactions as well as in processes with mutual nuclear excitation of the beam particles. We present results for the ρ 0 production cross section in Au-Au collisions at s N N = 200 GeV for coherent as well as incoherent coupling. The dependence of the cross section on the ρ 0 rapidity is compared to theoretical models. We also studied the ratio of coherent ρ 0 to direct π + π − production as well as the ρ 0 helicity matrix elements and we observe interference effects in the ρ 0 production. In addition STAR has measured the production of ρ 0 mesons in d -Au collisions at s N N = 200 GeV and that of e + e − -pairs in Au-Au at s N N = 200 GeV . We also see an enhancement around 1510 MeV / c 2 in π + π − π + π − final states in Au-Au collisions at s N N = 200 GeV .