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

Abstract

In an exposure of the Fermilab 15-foot neon-hydrogen bubble chamber to a quadrupole triplet neutrino beam, $49{\mu }^{-}{e}^{+}$ and $14{\mu }^{+}{e}^{-}$ events with $e^{\pm }$ momenta greater than 0.3 GeV/c have been observed, yielding $\mu e$ rates per charged-current event of (0.73±0.11)% and (1.1±0.3)%, respectively. The ${\mu }^{-}{e}^{+}$ 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 $14K_S^0$, $2\Lambda$, and $\frac{2\Lambda }{K_S^0}$ ambiguities, suggesting that the events are predominantly $D$-meson production and decay and that the ${{\Lambda }_c}^{+}\to \Lambda \mathrm{eX}$ branching ratio is very small. The corrected numbers of neutral strange particles per ${\mu }^{-}{e}^{+}$ and ${\mu }^{+}{e}^{-}$ event are 1.2±0.3 and ${0.6}_{-0.3\mathrm{}}^{+0.6\mathrm{}}$, respectively. Properties of the events, including strange-particle production, are compared to ${\mu }^{\mp }{\mu }^{\pm }$ events in the same experiment and to a charm production and decay model, and good agreement is found, apart from a possible enhancement at ∼5-6 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^2$ in the mass of the system recoiling against the ${\mu }^{+}$ in ${\mu }^{+}{e}^{-}$ (and ${\mu }^{+}{\mu }^{-}$) events. As reported previously, four events show short-lived-particle decays, and $D$-meson lifetime estimates are reevaluated using the final event sample. One ${\mu }^{+}{e}^{+}$ and three ${\mu }^{-}{e}^{-}$ events were observed. The ${\mu }^{-}{e}^{-}$ events are consistent with background and lead to a $\frac{{\mu }^{-}{e}^{-}}{{\mu }^{-}{e}^{+}}$ ratio of less than 0.07 (90% confidence level) for $e^{\pm }$ 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.