Rho meson photoproduction at low energies

Abstract

The $σ$-exchange and $f_2$-exchange mechanisms for $ρ$ meson photoproduction are re-examined. Then the commonly employed $σ$-exchange amplitude is revised by using the recent information from the analyses on the $ρ\to π^0π^0γ$ decay and the $σNN$ coupling constant from Bonn potential. Instead of relying on the Pomeron-$f$ proportionality assumption, the $f_2$ meson exchange amplitude is established from an effective Lagrangian which is constructed from the tensor structure of the $f_2$ meson. Phenomenological information together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ coupling constants. As a first step to improve the current theoretical models, we have also explored the effects due to the un-correlated $2π$ exchange amplitude with $πN$ intermediate state. This leading-order $2π$ exchange amplitude can be calculated using the coupling constants determined from the study of pion photoproduction and the empirical width of $ρ\to ππ$ decay. In comparing with the existing differential cross section data, we find that a model with the constructed $2π$, $σ$, and $f_2$ exchanges is comparable to the commonly used $σ$ exchange model in which the $σ$ coupling parameters are simply adjusted to fit the experimental data. We suggest that experimental verifications of the predicted single and double spin asymmetries in the small $|t|$ ($ < 2$ GeV$^2$) region will be useful for distinguishing the two models and improving our understanding of the non-resonant amplitude of $ρ$ photoproduction. Possible further improvements of the model are discussed.