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Muon-neutrino and -antineutrino scattering off electrons was detected in a 19-ton Al spark chamber, exposed to the wide-band v (F) beam from the CERN proton synchrotron. The background was determined experimentally. 11 (10) genuine v^-(F M -) e scattering events were found. The respective cross sections are (l.l±0.6)xl(T 42 (E v /GeV) cm 2 and (2.2±1.0)xlO~i 2 (E v /GeV) cm 2 . The analysis excludes a pure V~A interaction, and makes a pure V or A theory improbable. The data agree well with the Salam-Weinberg model and sin 2 0 w = 0.35±0.08.Measuring muon-neutrino-electron scattering (T)is one of the key experiments for testing unified gauge theories of weak and electromagnetic interactions. 1 These reactions are mediated by the neutral weak leptonic current only, 2 in contrast to v e e scattering, 3 which would also proceed by the usual charged-current V -A coupling. 4 The scattering of muon antineutrinos off electrons was discovered in the heavy-liquid bubble chamber Gargamelle. 5 The final result 6 is based on three v u e candidates. The present experiment measured both v^e scattering. 7 ' 8The v u -(z^-) e scattering cross section is, 9 in a general V 9 A framework, and for neutrino energies(2)withy =E e /E vs and a e = 17.2xio'^O^/GeV) cm 2 . The total cross sections are a=cr e (A+JB/3), o=a e (A/3+B).(3)Their ratio r e =v/v and also the average energy transfers, (y) and (~y), depend on the ratio A/B only. The singular case of a parity-conserving V (or A) electron current implies A =B, and thus equality of all i^ and i > M data 0 The minimal gauge model 10 has only one free parameter, x w = sin 2 0 w , which fixes A = (-i+*«,)*, &ndB=x w 2 . The apparatus was exposed to the neutrino beam, derived from the CERN proton synchrotron (PS)o A high-intensity (~ 10 13 protons per pulse), 26-GeV proton beam was steered onto an external targeto Focusing of the emitted secondaries resulted in a fairly pure v^ (V^) beam, with 0,2% (0<,3%) wrong-type v^9 and 0.4% v e (0.3% V e ) contamination The v (V) spectrum has its maximum energy at 1.5 (1.4) GeV, and an average of 2.2 (2 o 0) GeV. Absolute neutrino fluxes are known with an average uncertainty near 10% 0The optical spark chamber 7 ' 8 consisted of 1-cmthick aluminum plates, with an effective mass of 19 tons. The chamber was fired every PS radiation pulse. Two views were photographed, with a stereo angle of 90°. Since the average radiation length is only 22 cm, electrons in the GeV range are easily identified by the electromagnetic shower they induce. But also y rays initiate such showers, and their pattern cannot be distinguished from an electron-induced shower. The shower energy was measured by spark counting. An absolute calibration has been obtained from exposing part of the chamber to a monochromatic electron beam. In the energy range 0.1-2 GeV the electron energy E Q corresponding to N observed sparks is well described by E e =6.5AT/(l-0.0025iV). MeV. The resolution in energy and projected angle were found to be A£ e /£ e =22% and A0 proj * 12/ JE* mrad (JE* in G...

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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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