Measurement of elastic muon-neutrino scattering off protons

Single recoil protons have been detected in a multiplate Al spark chamber exposed to the 2-GeV wideband 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+or-15 events) and single-pion contribution (45+or-6 events), the final sample contains 62+or-19 genuine neutrino-induced single protons. This yields an effective ratio of neutral-current (NC) to charged-current (CC) events of R/sub N/=(15+or-5)%, in the range 0.2<-q/sup 2/<1.0 (GeV/c)/sup 2/. This number is due to a mixture of elastic neutrino scatterings off protons and neutrons. From the probability f/sub np/ for a recoil neutron to give an accepted proton, one derives a model-independent combination of the NC/CC ratios R/sub p/+f/sub np/R/sub n/, with f/sub np /=0.31+or-0.04. This favors axial-vector-isovector-dominant NC coupling constants and is consistent with the Weinberg-Salam model with sin/sup 2/ theta /sub W/=0.29/sub -0.11//sup +0.21/. In terms of this model, this corresponds to R/sub p/=(10+or-3)% and R/sub n/=(15 /sub -5//sup +3/)%. (16 refs).