[en] The neutrino data from Fermilab have been searched for events with unusual topologies involving muons. Neutrino events with two muons in the final state have been observed in two independent experiments. From all indications this represents a very exciting development and very likely represents new particle production. These events do not appear to be easily explainable from known sources such as π or K-decay. Also, a threshold seems indicated by the lack of observation of such 2μ events at Serpukhov
Dobson, P.N. Jr. (ed.); p. 252-281; 1976; Univ. of Hawaii Press; Honolulu; 6. topical conference in particle physics; Honolulu, Hawaii, USA; 6 Aug 1975