Measurement of Muon-Neutrino and -Antineutrino Scattering off Electrons

Abstract

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...

The Structure of Neutral Currents

“…We also show in Table (3.1) and (3.2) earlier data from GGM (Blietschau et al, 1978;Armenise et al 1979 a), 15 AP (Faissner et al, 1978); CB (Cnops et al 1978) and BEBC-TST (Armenise et al 1979 b).…”

A descoberta das correntes neutras das interações fracas

“…(8) [104], ou em reações puramente leptônicas como ν µ + e − → ν µ + e − [105,106], e também ν µ + e − →ν µ + e − [107], ou mesmoν e + e − → ν e + e − [108]. Ainda mais, em 1974, no FERMILAB, além de serem observadas as reações da Eq.…”

Coherent Conversion of Very Light Pseudoscalar Bosons

“…(4) The coherent a>°-exchange process is negligible at low energy, but the cross section grows strongly with energy up to p ~R A " 1 , reaching a value of the order of 10" 35 cm 2 within the diffraction peak (pR A 6< 1) on Al, for example. Since the single photon looks like a recoil electron in counter and in spark-chamber experiments 12 with v^ of a few GeV, events of coherent conversion a°-+y arising from the presence of a 0 in the neutrino beam could measurably contaminate v^e scattering in the near-forward direction, the cross section for To illustrate this we have taken the conditions of the experiment of Faissner etal., 12 folding the hypothetical-a 0 energy spectrum with Eq. (4).…”

“…With the axion spectrum we use here, the angular distribution of coherent conversion alone does not explain the data of Faissner etal. 12 Further, the v^e analysis involved different cuts, 12 and will be repeated taking possible axion effects into account. 13 In heavier target material the co-herent nuclear process would give further-enhanced event rates relative to neutrino-electron scattering because, although the density of nuclei is reduced from that of electrons by 1/Z, the coherent cross sections go as Z 2 (Primakoff effect) or A 2 , in the near-forward direction.…”

“…14 These can give rise to a further background at the level of neutrino-electron scattering. 12 However, as (at least) threeparticle final states, the energy spectrum of the photons is not that of the incident beam, and the extreme forward concentration, characteristic of the Primakoff effect in particular, is absent. An excess of apparent v^-(V^-) e scattering at the smallest angles would thus be expected from the presence of a 0 in the beam, which undergo coherent nuclear scattering into photons in the target.…”