f_2, sigma, and 2-pi exchanges in rho meson photoproduction

The $\sigma$ meson exchange model for $\rho$ photoproduction at low energies is re-examined and a new model is developed by considering explicit two-pion exchange and the $f_2$ tensor meson exchange. The $f_2$ exchange model, which is motivated by the low energy proton-proton elastic scattering, is constructed by fully taking into account of the tensor structure of the $f_2$ meson interactions. Phenomenological informations together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ meson's coupling constants. For $2\pi$ exchange, the loop terms including intermediate $\pi N$ and $\omega N$ channels are calculated using the coupling constants determined from the study of pion photoproduction. It is found that our model with $f_2$ and $2\pi$ exchanges can successfully replace the commonly used $\sigma$ exchange model that suffers from the big uncertainty of the coupling constants. We found that the two models can be distinguished by examining the single and double spin asymmetries.