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 m_μμ 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 → Ξμ ⁺μ^- decays.