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 Mμμ 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