Observation of Single-Pion Production by a Weak Neutral Current

Abstract

In exposures of the Argonne National Laboratory 12-ft bubble chamber filled with hydrogen and deuterium to a neutrino beam, we have observed events consisting of (1) a single ${\ensuremath{\pi}}^{+}$ meson originating in the liquid, and (2) a proton with an ${e}^{+}{e}^{\ensuremath{-}}$ pair pointing to it. Only a small fraction of these events can be ascribed to known reactions such as $\mathrm{np}\ensuremath{\rightarrow}\mathrm{nn}{\ensuremath{\pi}}^{+}$ and $\mathrm{np}\ensuremath{\rightarrow}\mathrm{np}{\ensuremath{\pi}}^{0}$. The remaining events, which correspond to a signal of about 4.5 standard deviations, we ascribe to the reactions $\ensuremath{\nu}p\ensuremath{\rightarrow}\ensuremath{\nu}n{\ensuremath{\pi}}^{+}$ and $\ensuremath{\nu}p{\ensuremath{\pi}}^{0}$.