The STAR Collaboration reports on the photoproduction of ${\pi }^{+}{\pi }^{-}$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly real photon emitted by one ion scatters from the other ion.

We fit the ${\pi }^{+}{\pi }^{-}$ invariant-mass spectrum with a combination of ${\rho }^0$ and $\omega$ resonances and a direct ${\pi }^{+}{\pi }^{-}$ continuum. This is the first observation of the $\omega$ in ultraperipheral collisions, and the first measurement of $\rho \text{−}\omega$ interference at energies where photoproduction is dominated by Pomeron exchange. The $\omega$ amplitude is consistent with the measured $\gamma p\to \omega p$ cross section, a classical Glauber calculation, and the $\omega \to {\pi }^{+}{\pi }^{-}$ branching ratio. The $\omega$ phase angle is similar to that observed at much lower energies, showing that the $\rho \text{−}\omega$ phase difference does not depend significantly on photon energy.

The ${\rho }^0$ differential cross section $d\sigma /dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with two minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

• Received 18 May 2017

DOI:https://doi.org/10.1103/PhysRevC.96.054904