  The HyperCP (E871) experiment collected several billion hyperon decays in the
1997 and 1999 Fermilab fixed-target running periods. Using the data from the
1999 run, we report on the observation of three isolated events with
reconstructed masses consistent with the hypothesis Sigma-plus -> proton +
mu-plus + mu-minus. This is the rarest baryon decay ever observed. The dimuon
mass distribution is unexpectedly narrow, suggesting the decay may proceed via
an intermediate state of mass 214.3 +/- 0.5 MeV/c^2. This state is consistent
with a short-lived pseudoscalar sgoldstino with parity-conserving interactions
decaying into two unlike-sign muons.
