$\rho^0$ photoproduction in ultraperipheral relativistic heavy ion collisions at $\sqrt{s_{NN}}$ = 200 GeV
Phys.Rev.C 77 (2008) 034910, 2008.
The STAR collaboration

Abstract (data 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 $\rho^{0}$ 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 $\sigma(AuAu \rightarrow Au^{*}Au^{*} \rho^{0})$ = 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.

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