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 rho(0)(770) mesons in exclusive reactions as well as in processes with mutual nuclear excitation of the beam particles. We present results for the rho(0) production cross section in Au-Au collisions at root S-NN = 200 GeV for coherent as well as incoherent coupling. The dependence of the cross section on the rho(0) rapidity is compared to theoretical models. We also studied the ratio of coherent rho(0) to direct pi(+)pi(-) production as well as the rho(0) helicity matrix elements and we observe interference effects in the rho(0) production. In addition STAR has measured the production of rho(0) mesons in d-Au collisions at root S-NN = 200 GeV and that of e(+)e(-)-pairs in Au-Au at root S-NN = 200 GeV. We also see an enhancement around 1510 MeV/c(2) in pi(+)pi(-)pi(+)pi(-) final states in Au-Au collisions at root S-NN = 200 GeV.