Decay Σ⁺→pℓ⁺ℓ^- within the standard model

The HyperCP collaboration has recently reported the observation of three events for the decay Σ⁺→pμ⁺μ^-. They have suggested that new physics may be required to understand the implied decay rate and the observed M_μμ 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 Σ⁺→pγ measurements, and estimate the other two with vector-meson-dominance models. Taking into account constraints from Σ⁺→pe⁺e^-, we find that Σ⁺→pμ⁺μ^- is long-distance dominated and its rate falls within the range suggested by the HyperCP measurement.