Abstract

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-omega interference at energies where photoproduction is dominated by Pomeron exchange. The omega amplitude is consistent with the measured gamma p -> omega p cross section, a classical Glauber calculation, and the omega -> pi(+) pi(-) branching ratio. The omega phase angle is similar to that observed at much lower energies, showing that the rho-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.