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\ifmmode\pm\else\textpm\fi{}15 events) and single-pion contribution (45\ifmmode\pm\else\textpm\fi{}6 events), the final sample contains 62\ifmmode\pm\else\textpm\fi{}19 genuine neutrino-induced single protons. This yields an effective ratio of neutral-current (NC) to charged-current (CC) events of ${R}_{N}=(15\ifmmode\pm\else\textpm\fi{}5)%$, in the range $0.2l\ensuremath{-}{q}^{2}l1.0$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. This number is due to a mixture of elastic neutrino scatterings off protons and neutrons. From the probability ${f}_{\mathrm{np}}$ for a recoil neutron to give an accepted proton, one derives a model-independent combination of the NC/CC ratios ${R}_{p}+{f}_{\mathrm{np}}{R}_{n}$, with ${f}_{\mathrm{np}}=0.31\ifmmode\pm\else\textpm\fi{}0.04$. This favors axial-vector-isovector-dominant NC coupling constants and is consistent with the Weinberg-Salam model with ${{sin}^{2}\ensuremath{\theta}}_{W}={0.29}_{\ensuremath{-}0.11}^{+0.21}$. In terms of this model, this corresponds to ${R}_{p}=(10\ifmmode\pm\else\textpm\fi{}3)%$ and ${R}_{n}=({15}_{\ensuremath{-}5}^{+3})%$.