# Coherent diffractive photoproduction of $\rho^{0}$ mesons on gold nuclei at RHIC

## TL;DR

This paper reports the first observation of $
ho$ and $
ho- ext{omega}$ interference in ultra-peripheral gold-gold collisions at RHIC, revealing diffraction patterns and insights into nuclear scattering processes.

## Contribution

It presents the first measurement of $
ho- ext{omega}$ interference and diffraction patterns in ultra-peripheral heavy-ion collisions at RHIC energies.

## Key findings

- First observation of $
ho$ and $ ext{omega}$ interference in ultra-peripheral collisions.
- Diffraction pattern in $
ho^0$ cross section consistent with scattering from gold nuclei.
- Better agreement with a Glauber model without nuclear shadowing.

## 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 $\rho$ and $\omega$ resonances and a direct $\pi^+\pi^-$ continuum. This is the first observation of the $\omega$ in ultra-peripheral 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 $\gamma p\rightarrow \omega p$ cross section, a classical Glauber calculation and the $\omega\rightarrow\pi^+\pi^-$ 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 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.