Abstract
Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ0 and direct π+π- photoproduction in ultraperipheral relativistic heavy ion collisions at sNN=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au*Au*ρ0)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ0 transverse momentum spectrum (pT2) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σinc/σcoh=0. 29±0.03(stat.)±0.08(syst.). The ratio of direct π+π- to ρ0 production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.