Observation of Elastic Antineutrino-Proton Scattering

We have observed 22 events of the process $\overline{\nu }p\to \overline{\nu }p$ with a background expectation of 8 events. The neutral-current-to-charged-current ratio $\frac{\sigma (\overline{\nu }p\to \overline{\nu }p)}{\sigma (\overline{\nu }p\to {\mu }^{+}n)}$ is measured to be 0.2±0.1 for $0.3<\mathrm{}{q}^2<0.9\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$, where $-q^2$ is the square of the four-momentum transfer to the proton. When combined with our previous measurement of $\nu p\to \nu p$, this yields a ratio $\frac{\sigma (\overline{\nu }p\to \nu p)}{\sigma (\nu p\to \nu p)}=0.4\mathrm{}\pm 0.2$, where the error is statistical, suggesting that a purely vector, neutral, hadronic current is unlikely.