The actual detection was thus only that of an electron (positron) and an antimuon (muon), i.e., two different leptons with opposite charges, which apparently violates the law of lepton number conservation.    From Wordnik.com.  [Additional background material on the Nobel Prize in Physics 1995] Reference

The first indication of a possible new phenomenon was that the research team observed 24 events of the type electron + positron - electron + antimuon + i.p. or electron + positron - positron + muon + i.p. where i.p. stands for invisible particles; those that left no trace in the detector.    From Wordnik.com.  [Press Release: The 1995 Nobel Prize in Physics] Reference

(or positron) and an antimuon (or muon) with the opposite sign on its charges were detected.    From Wordnik.com.  [Press Release: The 1995 Nobel Prize in Physics] Reference