U. Dore

Precision measurement of electroweak parameters from the scattering of muon-neutrinos on electrons

A b s t r a c t We report final results on electroweak parameters from muon-neutrino electron scattering o b s e r v e d in the C H A R M II detector from 1987 till 1991. In total 2677 + 82 and 2752 ± 88 neutrino-electron scattering events have been d e t e c t e d in the v and ~ - b e a m , respectively. From the ratio o f differential cross sections we obtain for the electroweak mixing angle sin 2 ®,,, = 0.2324 4- 0.0083. From the absolute neutrino-electron scattering event rate w e d e t e r m i n e d the effective vector and axial-vector neutral current coupling constants to be g~, = - 0 . 0 3 5 4- 0.017 and g~ = - 0 . 5 0 3 -~ 0.017.

A precise determination of the electroweak mixing angle from semi-leptonic neutrino scattering

Abstract The cross section ratio of neutral-current and charged-current semi-leptonic interactions of muon-neutrinos on isoscalar nuclei has been measured with the result: R v =0.3093±0.0031 for hadronic energy larger than 4 GeV. From this ratio, the electroweak mixing angle is determined to be sin 2 θ w =0.236+0.012( m c −1.5)±0.005 (exp.)±0.003 (theor.), where m c is the charm-quark mass in GeV/ c 2 .

A search for decays of heavy neutrinos in the mass range 0.5–2.8 GeV

A search for decays of heavy neutrinos was conducted by the CHARM Collaboration in a prompt neutrino beam produced by dumping 400 GeV protons in a Cu target, and in the CERN wide-band neutrino beam produced by 400 GeV primary protons. No candidate event was found. In the beam-dump experiment heavy neutrinos have been assumed to be produced by mixing in charmed D meson decays. Neutrinos decaying into e+e−ve, μ+e−vμ, and μ+μ−vμ were searched for. Limits of |Uei|2, |Uμi|2 < 10−7 were obtained for neutrino masses around 1.5 GeV. In the wide-band experiment heavy neutrinos were assumed to be produced by neutral-current neutrino interactions in the CHARM calorimeter. Here a search was made for neutrinos decaying into a μ and hadrons. This experiment is sensitive to decays of neutrinos with mass in the range 0.5–2.8 GeV with limits of |Uμi|2 < 3 × 10−4 for masses around 2.5 GeV. These measurements extend our previous results in the mass range 10–400 MeV.

A Precise Determination of the Electroweak Mixing Angle from Semileptonic Neutrino Scattering

The cross-section ratio of neutral-current and charged-current semileptonic interactions of muon-neutrinos on isoscalar nuclei has been measured with the result:Rv=0.3093±0.0031 for hadronic energy larger than 4 GeV. From this ratio we determined the electroweak mixing angle sin2θW, wheremc is the charm-quark mass in GeV/c2. Comparison with direct measurements ofmw andmz determines the radiative shift of the intermediate boson mass Δr=0.077±0.025(exp.)±0.038(syst.), in agreement with the prediction. Assuming the validity of the electroweak standard theory we determined ϱ=0.990−0.013(mc−1.5)±0.009(exp.)±0.003(theor.).

A search for decays of heavy neutrinos

Abstract A search for heavy neutrinos was conducted in the neutrino beam produced by the 400 GeV proton beam-dump and in the 400 GeV wide-band neutrino beam at CERN. A heavy neutrino associated with the τ lepton was searched for in the beam-dump experiment. No assumption on the nature of heavy neutrinos was made in the wide-band beam experiment. A search was made for neutrinos decaying into two electrons and a light neutrino. Since no events were observed, an upper limit on the neutrino mixing angles as a function of the neutrino mass is derived. The beam-dump experiment is sensitive to a neutrino mass range of 10–250 MeV and an upper limit at 90% confidende level on the square of the neutrino mixing angle of θ10 −10 was obtained for neutrino masses greater than 180 MeV. The wide-band neutrino experiment is sensitive to neutrinos with a mass in a larger range (10–490 MeV), and for masses greater than 250 MeV yields an upper limit on the square of the neutrino mixing angle of θ10 −6 .

Search for axion-like particle production in 400 GeV proton-copper interactions

Abstract A search for axion-like particles was performed at the 400 GeV proton beam-dump experiment at CERN. Exploring an empty decay region of 35 m length and 9 m 2 cross section, we searched for decays of neutral and penetrating scalar particles into a pair of photons, electrons or muons. No evidence for the existence for such particles was found in this experiment. Limits are quoted as a function of the mass and of the model independent decay constant of axions.

Experimental study of the nucleon structure functions and of the gluon distribution from charged-current neutrino and antineutrino interactions

Abstract Inclusive neutrino and antineutrino charged current interactions were studied in the CHARM detector exposed to neutrino and antineutrino Wide Band Beams of the CERN 400 GeV SPS. The x and Q2 dependence of the structure functions F2 and xF3 and of the antiquark momentum distribution q were determined. The data have been interpreted in terms of QCD theory using the Furmanski-Petronzio method. In this way we have determined ΛLO = [190−40+70 (stat) ± 70 (syst.)] MeV and the structure functions of quarks and gluons without specific assumptions on their analytic dependence. The results agree with previous experiments which relied on model assumptions in the analysis. We conclude that the model independent simultaneous analysis of the xF3, F2, q structure functions gives a more reliable determination of the gluon distribution in the nucleon.

A search for oscillations of muon neutrinos in an experiment with L/E ≅0.7 km/GeV

Abstract A low-energy muon neutrino beam was used to illuminate two similar fine-grained neutrino detectors placed at 123 m and 903 m from the beginning of the neutrino source. The fiducial masses of the “close” and the “far” detector were about 36 tons and 120 tons respectively. The average energy of the selected neutrino events was 1.5 GeV. Data were recorded for an integrated flux of about 1019 protons. Quasielastic charged current events initiated by νes have been searched for and an upper limit of 2.7%, at 90% confidence level, is obtained for the fraction of νμs transformed into νes. For complete mixing this correspnds to a limit Δm2 ⩽ 0.20 eV2 for the transition νμ → νe and to a minimum value of the mixing parameter sin22θ = 0.04. In the same data sample we compared the rates in the two detectors of νμ-initiated charged current events, mainly of quasielastic type: we conclude that oscillations of νμs to ντs, and possibly heavier neutrinos, do not appear. For complete mixing the limit in this case is Δm2 ⩽ 0.29 eV2.

Comparison of the reaction pp → K0K0 at rest in hydrogen and deuterium. A test of S-state annihilation dominance

From the comparison of the frequency of antiproton annihilation at rest in two mesons in H2 and D2, the dominance of S-wave in pp annihilation at rest is confirmed, the P-wave contribution being less than 6% at the 95% confidence level. The apparent discrepancy between the channels pp→K0K0 and pp→ππ is due to the strong dependence of the annihilation dynamics from the angular momentum.

Neutral current coupling constants from neutrino- and antineutrino-electron scattering

Abstract We are reporting on a determination of neutral current coupling constants from a study of differential cross sections of muon- (anti)neutrino-electron scattering. The results were obtained with the CHARM-II detector which was exposed to the CERN wide band neutrino beam in the years 1987–1990. A total of about 2100 ve and 2200 ve scattering events were observed. From a comparison between the data and predicted event distributions, the effective vector and axial-vector neutral current coupling constants of the electron were determined to be g e v =−0.025±0.019 and g e A =−0.503±0.018.