Dilepton production by neutrinos in the Fermilab 15-foot bubble chamber

In an exposure of the Fermilab 15-foot neon-hydrogen bubble chamber to a quadrupole triplet neutrino beam, $49{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ and $14{\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$ events with ${e}^{\ifmmode\pm\else\textpm\fi{}}$ momenta greater than 0.3 GeV/c have been observed, yielding $\ensuremath{\mu}e$ rates per charged-current event of (0.73\ifmmode\pm\else\textpm\fi{}0.11)% and (1.1\ifmmode\pm\else\textpm\fi{}0.3)%, respectively. The ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ rate shows no strong energy dependence in the range from 30 to 300 GeV. The 18 neutral strange particles observed in the 63 events contain $14{K}_{S}^{0}$, $2\ensuremath{\Lambda}$, and $\frac{2\ensuremath{\Lambda}}{{K}_{S}^{0}}$ ambiguities, suggesting that the events are predominantly $D$-meson production and decay and that the ${{\ensuremath{\Lambda}}_{c}}^{+}\ensuremath{\rightarrow}\ensuremath{\Lambda}\mathrm{eX}$ branching ratio is very small. The corrected numbers of neutral strange particles per ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}$ and ${\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$ event are 1.2\ifmmode\pm\else\textpm\fi{}0.3 and ${0.6}_{\ensuremath{-}0.3}^{+0.6}$, respectively. Properties of the events, including strange-particle production, are compared to ${\ensuremath{\mu}}^{\ensuremath{\mp}}{\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}$ events in the same experiment and to a charm production and decay model, and good agreement is found, apart from a possible enhancement at \ensuremath{\sim}5-6 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$ in the mass of the system recoiling against the ${\ensuremath{\mu}}^{+}$ in ${\ensuremath{\mu}}^{+}{e}^{\ensuremath{-}}$ (and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$) events. As reported previously, four events show short-lived-particle decays, and $D$-meson lifetime estimates are reevaluated using the final event sample. One ${\ensuremath{\mu}}^{+}{e}^{+}$ and three ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{\ensuremath{-}}$ events were observed. The ${\ensuremath{\mu}}^{\ensuremath{-}}{e}^{\ensuremath{-}}$ events are consistent with background and lead to a $\frac{{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{\ensuremath{-}}}{{\ensuremath{\mu}}^{\ensuremath{-}}{e}^{+}}$ ratio of less than 0.07 (90% confidence level) for ${e}^{\ifmmode\pm\else\textpm\fi{}}$ momenta above 0.8 GeV/c. Five candidates for dilepton production by electron neutrinos and antineutrinos in the beam are consistent with approximately 1% rates. No good three-lepton candidates were found, and one, previously reported, four-lepton candidate was found.