s→dγcontribution to hyperon radiative decays: A QCD-sum-rule approach

We analyze, using the method of the QCD sum rules, the short-distance contribution $s\to d\gamma$ to the hyperon radiative decays ${\Sigma }^{+}\to p\gamma$ and ${\Xi }^{-}\to {\Sigma }^{-}\gamma$. A more reliable estimate of this contribution is obtained, giving branching ratios that are larger than obtained before for the same process, although still small when compared with experimental data. The asymmetry parameter $\alpha$ turns out to depend on the decay considered and surprisingly is equal to -1 for ${\Sigma }^{+}\to p\gamma$, for dynamical reasons which we discuss at length.