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Coherent diffractive photoproduction of $ρ^{0}$ mesons on gold nuclei at RHIC

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. We fit the $π^+π^-$ invariant mass spectrum with a combination of $ρ$ and $ω$ resonances and a direct $π^+π^-$ continuum. This is the first observation of the $ω$ in ultra-peripheral 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 $γp\rightarrow ωp$ cross section, a classical Glauber calculation and the $ω\rightarrowπ^+π^-$ 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 $ρ^0$ differential cross section $dσ/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 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.