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 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+ 15 events) and single-pion contribution (45 + 6 events), the final sample contains 6 2 k 19 genuine neutrinoinduced single protons. This yields an effective ratio of neutral-current (NC) to charged-current (CC) events of R, = (15+5)%, in the range 0.2< --q '< 1.0 (~eV/c)'. This number is due to a mixture of elastic neutrino scatterings off protons and neutrons. From the probability f,, for a recoil neutron to give an accepted proton, one derives a model-independent combination of the NC/CC ratios R, + f,,R,, with f,, = 0.3 1 10.04. This favors axial-vector-isovector-dominant NC coupling constants and is consistent with the Weinberg-Salam model with sin20w =0.29?:::;.In terms of this model, this corresponds to R, = (10&3)% and R, = (151:)%. I. INTRODUCTIONThe absolute r a t e s f o r reactions (1) and (2) a r e difficult to measure. Experimentally the Elastic scattering of high-energy muon neuratios R, and R, a r e l e s s sensitive to the selectrinos v, (' F,) off p r o t o n s p and neutrons n ranks tion and acceptance criteria. among the fundamental p r o c e s s e s of weak-inter-F i r s t limits on the ratio R, c a m e f r o m early action physics experiments a t CERN.4 Eventually, elastic neutrino-proton scattering w a s discovered by the v p + p ' l J~+ P , Harvard-Pennsylvania-Wisconsin collaboration -v,+p-7,+p. (T) (HPW),5 and by a Columbia-Illinois-Rockefeller (CIR) group.6 A subsequent measurement w a s (2) made by the Gargamelle7 collaboration, and a (3) preliminary result f r o m our experiment w a s published in 1977.8 The present paper r e p o r t s They yield significant information about the strucour final result on R,, and describes in particut u r e of the hadronic weak neutral current (NC) l a r the method used t o disentangle v,p f r o m involved.' The simplicity of these reactions adv,n scattering. m i t s of a description in t e r m s of a s m a l l number of f o r m factors. The vector f o r m factors Fv and F, , a r e well known f r o m electron-nucleon ( e N ) 11. APPARATUS scattering. Some information exists about the The apparatus consisted of an a r r a y of 141 axial-vector f o r m factor FA f r o m studies of the optical spark chambers. ~a c h chamber had two charged-current (CC) quasielastic reactionz 1-cm-wide gaps filled with a Ne-He mixture.V, +n-k~-+p,-v,+p-p+ + n . (3) Different gauge theories tend to give different values for the c r o s s section of the NC elastic scattering processes. In the framework of the Weinberg-Salam model3 a l l NC p r o c e s s e s depend on one parameter only, sinzO,, and hence the ratios R D = a ( v f i -v , P ) /~(~,~-F-P) (4)and R, f o r the antineutrinos, as well a sThe aluminum plates w e r e 1 c m thick, -2 X 2m2 large. The last 12 chambers w e r e interl...

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