Low energy \omega (\to \pi^0 \gamma) meson photoproduction in the nucleus

Abstract

The $\pi^0 \gamma$ invariant mass distribution spectra in the $ (\gamma, \pi^0\gamma) $ reaction were measured by TAPS/ELSA collaboration to look for the hadron parameters of the $\omega$ meson in Nb nucleus. We study the mechanism for this reaction, where we consider that the elementary reaction in Nb nucleus proceeds as $ \gamma N \to \omega N; ~ \omega \to \pi^0\gamma $. The $\omega$ meson photoproduction amplitude for this reaction is extracted from the measured four momentum transfer distribution in the $ \gamma p \to \omega p $ reaction. The propagation of the $\omega$ meson and the distorted wave function for the $\pi^0$ meson in the final state are described by the eikonal form. The $\omega$ and $\pi^0$ mesons nucleus optical potentials, appearing in the $\omega$ meson propagator and $\pi^0$ meson distorted wave function respectively, are estimated using the "$t\varrho$" approximation. The effects of pair correlation and color transparency are also studied. The calculated results do not show medium modification for the $\omega$ meson produced in the nucleus for its momentum greater than 200 MeV. It occurs since the $\omega$ meson dominantly decays outside the nucleus. The dependence of the cross section on the final state interaction is also investigated. The broadening of the $\omega$ meson mass distribution spectra is shown to occur due to the large resolution width associated with the detector used in the experiment.