Measurement of Elastic Muon Neutrino Scattering Off Protons

Abstract

Single recoil protons have been detected in a multiplate Al spark chamber exposed to the 2-GeV wide-band neutrino beam from the CERN proton synchrotron. Neutron-induced protons were suppressed by suitable geometrical and kinematical cuts. After correction for remaining neutron background (110±15 events) and single-pion contribution (45±6 events), the final sample contains 62±19 genuine neutrino-induced single protons. This yields an effective ratio of neutral-current (NC) to charged-current (CC) events of RN=(15±5)%, in the range 0.2<−q2<1.0 (GeV/c)2. This number is due to a mixture of elastic neutrino scatterings off protons and neutrons. From the probability fnp for a recoil neutron to give an accepted proton, one derives a model-independent combination of the NC/CC ratios Rp+fnpRn, with fnp=0.31±0.04. This favors axial-vector-isovector-dominant NC coupling constants and is consistent with the Weinberg-Salam model with sin2θW=0.29−0.11+0.21. In terms of this model, this corresponds to Rp=(10±3)% and Rn=(15−5+3)%.