Photoproduction in Ultra-Peripheral Relativistic Heavy Ion Collisions at STAR

Abstract

In ultra-peripheral relativistic heavy ion collisions the beam ions scatter at impact parameters larger than the sum of their radii, so that they interact via long range electromagnetic forces. Due to the Lorentz-boost of the beam particles, the exchanged virtual photons have high energies and can induce the photoproduction of vector-mesons. We present recent results of the STAR experiment at RHIC on $\rho^0(770)$ production in Au-Au ultra-peripheral collisions at various energies. STAR has also observed the photoproduction of $\pi^+\pi^-\pi+\pi^-$, which is related to the still poorly known excited states of the $\rho^0$.