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 $\rho^0(770)$ 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 $\sqrt{s_{NN}}=200$ 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 $\sqrt{s_{NN}}=200$ GeV and that of $e^+e^-$-pairs in Au-Au at $\sqrt{s_{NN}}=200$ GeV. We also see an enhancement around 1510 MeV/c in $\pi^+\pi^-\pi^+\pi^-$ final states in Au-Au collisions at $\sqrt{s_{NN}}=200$ GeV.