Low-energyω(→π0γ)meson photoproduction in the nucleus

Abstract

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