We present a summary of recent photoproduction results in ultra peripheral relativistic heavy ions collisions with STAR. These collisions have impact parameters larger then twice the nuclear radius; the nuclei do not physically collide, but interact via long-range electromagnetic fields. We observe exclusive ρ0 production as well as AuAu → Au*Au*ρ0 with accompanying mutual nuclear excitation at sqrt(sN N) = 200 GeV. We report the ρ0 production cross section for both coherent and incoherent coupling accompanied by mutual nuclear excitation. We have studied the cross section as a function of pT, yρ0 and Mππ and compared it to theoretical models. In addition, we measured the ρ0 helicity matrix elements. They are found to be consistent with s-channel helicity conservation. The ratio of coherent ρ0 and direct π+π- pair photoproduction has been measured and found to be consistent with earlier measurements. The 4-pion final state AuAu → π+π-π+π- state has also been observed.
All Science Journal Classification (ASJC) codes
- Atomic and Molecular Physics, and Optics
- Nuclear and High Energy Physics