New Results in Vector Meson Dominance and Rho Meson Physics

Recent results by Benayoun et al. comparing the predictions of a range of existing models based on the Vector Meson Dominance hypothesis are summarized and discussed. These are compared with data on ee → ππ and ee → μμ cross-sections and the phase and near-threshold behavior of the timelike pion form factor, with the aim of determining which (if any) of these models is capable of providing an accurate representation of the full range of experimental data. We find that, of the models considered, only Hidden Local Symmetry (HLS) model proposed by Bando et al. is able to consistently account for all information, provided one allows its parameter a to vary from the usual value of 2 to 2.4. Our fit with the HLS model gives a point–like coupling γππ of magnitude ≃ −e/6, while the common formulation of VMD excludes such a term. The resulting values for the ρ mass and ππ and ee partial widths as well as the branching ratio for the decay ω → ππ obtained within the context of this model are consistent with previous results.