Photoproduction in Ultra-Peripheral Heavy Ion Collisions at STAR

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 ρ0(770) mesons in exclusive reactions as well as in processes with mutual nuclear excitation of the beam particles. We present results for the ρ0 production cross section in Au-Au collisions at for coherent as well as incoherent coupling. The dependence of the cross section on the ρ0 rapidity is compared to theoretical models. We also studied the ratio of coherent ρ0 to direct π+π− production as well as the ρ0 helicity matrix elements and we observe interference effects in the ρ0 production. In addition STAR has measured the production of ρ0 mesons in d-Au collisions at and that of e+e−-pairs in Au-Au at . We also see an enhancement around 1510MeV/c2 in π+π−π+π− final states in Au-Au collisions at .