ρ0 photoproduction in ultraperipheral relativistic heavy ion collisions at collisions at √sNN =200 GeV

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of 0 and direct +- photoproduction in ultraperipheral relativistic heavy ion collisions √sNN =200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of (AuAuAu*Au*0)=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 0 transverse momentum spectrum (p) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find inc/coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct +- to 0 production is comparable to that observed in p collisions at HERA and appears to be independent of photon energy. Finally, the measured 0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.