Observation of Anomalous Dimuon Events in the NuTeV Decay Detector
  A search for long-lived neutral particles (N^0) which decay into at least one muon has been performed using an instrumented decay channel at the E815 (NuTeV) experiment at Fermilab. The decay channel was composed of helium bags interspersed with drift chambers, and was used in conjunction with the NuTeV neutrino detector to search for N^0 decays. The data were examined for particles decaying into the muonic final states mu mu, mu e, and mu pi. Three mu mu events were observed over an expected background of 0.040 +/- 0.009 events; no events were observed in the other modes. Although the observed events share some characteristics with neutrino interactions, the observed rate is a factor of 75 greater than expected. No Standard Model process appears to be consistent with this observation. 
NuTeV Collaboration: T. Adams, A. Alton, S. Avvakumov, L. de Barbaro,   P. de Barbaro, R.H. Bernstein, A. Bodek, T. Bolton, ...