Observation of prompt like-sign dimuon production in neutrino reactions

We report on the observation of 12 like-sign ( ..mu../sup -/..mu../sup -/) neutrino-induced dimuon events with muon momenta greater than 9 GeV. The background from ..pi.. and K decay is 1.3 events so that we conclude that this prompt signal is real with a significance greater than 1 in 10/sup 7/. Although the overall rate is higher than present theoretical estimates, the kinematic distributions of these events are qualitatively consistent with a picture of charm-anticharm production. The ratio of ..mu../sup -/..mu../sup -//..mu../sup -/ shows a strong energy dependence and rises to (2.5 +- 1.0) x 10/sup -3/ at E/sub ..nu../ = 250 GeV.