Observation of Prompt Like-Sign Dimuon Production in Neutrino Reactions

We report on the observation of twelve like-sign (${\mu }^{-}{\mu }^{-}$) 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 ${10}^7$. 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 $\frac{{\mu }^{-}{\mu }^{-}}{{\mu }^{-}}$ shows a strong energy dependence and rises to (2.5±1.0) × ${10}^{-3\mathrm{}}$ at $E_{\nu }=250\mathrm{}$ GeV.