  Photoproduction reactions occur when the electromagnetic field of a
relativistic heavy ion interacts with another heavy ion. The STAR collaboration
presents a measurement of rho^0 and direct pi^+pi^- photoproduction in
ultra-peripheral relativistic heavy ion collisions at sqrt(s_{NN})=200 GeV. We
observe both exclusive photoproduction and photoproduction accompanied by
mutual Coulomb excitation. We find a coherent cross-section of sigma(AuAu) ->
Au^*Au^*rho^0 = 530 pm 19 (stat.) pm 57 (syst.) mb, in accord with theoretical
calculations based on a Glauber approach, but considerably below the
predictions of a color dipole model. The rho^0 transverse momentum spectrum
(p_{T}^2) is fit by a double exponential curve including both coherent and
incoherent coupling to the target nucleus; we find sigma_{inc}/sigma_{coh} =
0.29 pm 0.03 (stat.) pm 0.08 (syst.). The ratio of direct pi^+pi^- to rho^0
production is comparable to that observed in gamma p collisions at HERA, and
appears to be independent of photon energy. Finally, the measured rho^0 spin
helicity matrix elements agree within errors with the expected s-channel
helicity conservation.
