Σ+ → pμ+μ−: Standard Model or New Particle?

The HyperCP collaboration reported last year the observation of three events for the decay Σ+ → pμ+μ−. They suggested that new physics may be required to understand the implied decay rate and the observed mμμ distribution. Motivated by this result we re‐examine this mode, both within the standard model and assuming there is a new particle. Within the SM we find that Σ+ → pμ+μ− is long‐distance dominated and its rate falls within the range suggested by the HyperCP measurement. If there is a new particle we find that it cannot be a scalar or a vector. If it is a pseudoscalar or axial vector it would lead to significantly enhanced rates in K → ππμ+μ− and Ω → Ξμ+μ− decays.