Observation of Elastic Antineutrino-Proton Scattering

We have observed 22 events of the process νp→νp with a background expectation of 8 events. The neutral-current-to-charged-current ratio ${{σ}({{ν}}p{→}{{ν}}p)}{{σ}({{ν}}p{→}{{μ}}⁺n)}is measured to be 0.2±0.1 for0.3<{q}²<0.9$ ${(GeV/c)}²$, where ${-}{q}²$ is the square of the four-momentum transfer to the proton. When combined with our previous measurement of ${ν}p{→}{ν}p$, this yields a ratio ${{σ}({{ν}}p{→}{ν}p)}{{σ}({ν}p{→}{ν}p)}=0.4±{}0.2$, where the error is statistical, suggesting that a purely vector, neutral, hadronic current is unlikely.