Coherent diffractive photoproduction of ρ0 mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider

The STAR Collaboration reports on the photoproduction of π⁺ π^- 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. In this work, we fit the π⁺ π^- invariant-mass spectrum with a combination of ρ⁰ and ω resonances and a direct π⁺ π^- continuum. This is the first observation of the ω in ultraperipheral collisions, and the first measurement of ρ - ω interference at energies where photoproduction is dominated by Pomeron exchange. The ω amplitude is consistent with the measured γp → ωp cross section, a classical Glauber calculation, and the ω → π⁺ π^- branching ratio. The ω phase angle is similar to that observed at much lower energies, showing that the ρ - ω phase difference does not depend significantly on photon energy. The ρ⁰ differential cross section dσ/dt exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with two minima visible. In conclusion, 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.