  The LHCb Collaboration has recently reported evidence for the rare hyperon
decay $\Sigma^+\to p\mu^+\mu^-$ that is consistent with the standard model
expectation. Motivated by this new result we revisit the calculation of this
mode including both long and short distance contributions. In the standard
model this mode is completely dominated by long distance physics and thus
subject to large uncertainties. We present a range of predictions for the rate,
the dimuon invariant mass spectrum, and a muon polarization asymmetry that
covers these uncertainties as well as known constraints. We study the interplay
between short and long distance contributions which could result in additional
asymmetries, but we find that they are negligible within the standard model. We
propose a parameterization of these asymmetries in terms of a couple of
constants that can arise from new physics.
