ρ0photoproduction in ultraperipheral relativistic heavy ion collisions atsNN=200GeV

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 ultraperipheral relativistic heavy ion collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of $\sigma (\mathrm{AuAu}\to {\mathrm{Au}}^{*}{\mathrm{Au}}^{*}{\rho }^0)=530\pm 19(\mathrm{stat}.)\pm 57(\mathrm{syst}.)$ 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 ${\sigma }_{\mathrm{inc}}/{\sigma }_{\mathrm{coh}}=0.29\pm 0.03(\mathrm{stat}.)\pm 0.08(\mathrm{syst}.)$. 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.