Weak radiative decays of hyperons: Quarks,SU(6)W, and vector-meson dominance

Quark-model, $\mathrm{SU}{\mathrm{}\left(6\right)\mathrm{}}_W$, and vector-dominance-model (VDM) methods are simultaneously used to investigate weak radiative decays of hyperons. The use of the VDM helps in the identification of the origin of the quark-model violation of the Hara theorem: besides the ${\overline{u}}_1{\sigma }_{\mu \nu }{\gamma }_5{q}^{\nu }{u}_2{A}^{\mu }$ term the $A_{\mu }{J}_V^{\mu }$ "contact" photon-quark interaction generates an additional contribution which is effectively equivalent to a nonvanishing ${\overline{u}}_1{\gamma }_5{\gamma }_{\mu }{u}_2{A}^{\mu }$ photon-hadron coupling. Our approach provides a $\mathrm{SU}{\mathrm{}\left(6\right)\mathrm{}}_W$-based symmetry connection between radiative and nonleptonic hyperon decays. As a result, a parameter-free symmetry prediction for asymmetries and branching ratios in weak radiative hyperon decays is obtained. The ${\Sigma }^{+}\to p\gamma$ decay asymmetry is found to be large and negative ($\alpha \simeq -0.6\mathrm{}$).