Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Coherent diffractive photoproduction of $\rho^{0}$ mesons on gold nuclei
  at RHIC   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.

26
The categories of this paper are: bayes-an
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
20
The categories of this paper are: acc-phys
4
The categories of this paper are: cond-mat
1
The categories of this paper are: cs
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
10
The categories of this paper are: q-bio
5
The categories of this paper are: gr-qc