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 ${\pi }^{+}$ meson originating in the liquid, and (2) a proton with an $e^{+}{e}^{-}$ pair pointing to it. Only a small fraction of these events can be ascribed to known reactions such as $\mathrm{np}\to \mathrm{nn}{\pi }^{+}$ and $\mathrm{np}\to \mathrm{np}{\pi }^0$. The remaining events, which correspond to a signal of about 4.5 standard deviations, we ascribe to the reactions $\nu p\to \nu n{\pi }^{+}$ and $\nu p{\pi }^0$.