DecayΣ+→pℓ+ℓ−within the standard model

The HyperCP collaboration has recently reported the observation of three events for the decay ${\Sigma }^{+}\to p{\mu }^{+}{\mu }^{-}$. They have suggested that new physics may be required to understand the implied decay rate and the observed $M_{\mu \mu }$ distribution. Motivated by this result, we reexamine this mode within the standard model, considering both the short-distance and long-distance contributions. The long-distance part depends on four complex form factors. We determine their imaginary parts from unitarity, fix two of the real parts from the ${\Sigma }^{+}\to p\gamma$ measurements, and estimate the other two with vector-meson-dominance models. Taking into account constraints from ${\Sigma }^{+}\to p{e}^{+}{e}^{-}$, we find that ${\Sigma }^{+}\to p{\mu }^{+}{\mu }^{-}$ is long-distance dominated and its rate falls within the range suggested by the HyperCP measurement.