Observation of elastic antineutrino-proton scattering

We have observed 22 events of the process anti νp→anti νp with a background expectation of 8 events. The neutral-current-to-charged-current ratio sigma (anti νp→anti νp)/sigma (anti νp→μ+n) is measured to be 0.2 +- 0.1 for 0.3<q2<0.9 (GeV/c)2, where -q2 is the square of the four-momentum transfer to the proton. When combined with our previous measurement of νp→νp, this yields a ratio sigma (anti νp→νp)/sigma (νp→νp) =0.4 +- 0.2, where the error is statistical, suggesting that a purely vector, neutral, hadronic current is unlikely