B. Borgia

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 .

Experimental study of neutral-current and charged-current neutrino cross sections

Abstract Samples of 9200 muon-neutrino and 3800 muon-antineutrino interactions on nuclei were obtained with the fine-grain calorimeter of the CHARM Collaboration at the CERN 200 GeV narrow-band neutrino beam. The interactions were classified as either neutral-current or charged-current processes on an event-by-event basis. Neutral-current and charged-current cross sections in neutrino and antineutrino interactions are presented. From these results we deduce a statistically significant contribution of right-handed coupling to the neutral hadronic current, and a value of the electroweak mixing angle corresponding to sin 2 θ = 0.220 ± 0.014.

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.

Experimental study of opposite-sign and same-sign dimuon events produced in wide-band neutrino and antineutrino beams

Abstract We report on a study of 495 ± 32 ν μ -induced and 285 ± 29 ν μ - induced prompt opposite-sign dimuon events, obtained in the CERN Super Proton Synchrotron (SPS) horn-focused beam, using the CHARM neutrino detector. Prompt events are selected using the observed distribution of the distance between the two muons at the vertex. The observed increase of the ratio of prompt dimuon to single muon events with E vis is compatible with the expected behaviour for charm quark production and decay. Assuming the GIM mechanism, we fond a strange-sea contribution of 0.050 ± 0.015, in agreement with our previous result not involving the charm-changing currents. In the same sample we observed 74 ± 17 ν μ -induced and 52 ± 13 ν μ - induced prompt same-sign dimuon events. On the basis of their kinematics, these events can be well described as being of hadronic origin. The ratios μ − μ − /μ − μ + and μ + μ + /μ + μ − are found to be 0.14 ± 0.04 (stat.) ± 0.03 (syst.) and 0.18 ± 0.06 (stat.) ± 0.03 (syst.), respectively. The observed ratio σ(μ + μ + ) σ(μ − μ − ) = 0.50 ± 0.20 is compatible with the ratio of the single-muon charged-current cross sections, σ( ν μ N → μ + X ) σ(ν μ N → μ − X ) = 0.498 ± 0.019 .

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.

Experimental study of differential cross sections in charged-current neutrino and antineutrino interactions

Abstract We present differential cross sections d σ /d y corrected for resolution and acceptance, for events induced by both the neutral- and charged-current interactions of neutrinos and antineutrinos. They are based on 8553 neutrino and 3578 antineutrino events obtained using the CHARM fine-grain calorimeter in the CERN 200 GeV narrow-band beam. From these differential cross sections we demonstrate that the coupling strength of the weak neutral current to the strange quark is compatible with being equal to that of the down quark. Assuming this equality we then describe the weak neutral current in terms of one parameter, sin 2 θ , which we find to be 0.222 ± 0.016. The charged-current differential cross sections yield values of the fractional momentum-weighted content of the nucleon for non-strange (0.12 ± 0.04) and strange (0.06 ± 0.04) sea quarks. Furthermore, from the strength of the allowed y 2 term in the neutral-current differential cross sections we put a limit of 3% on the presence of scalar or pseudoscalar contributions to the weak neutral current. This can alternatively be expressed in terms of the Callan-Gross violation parameter R , where we find R = 0 .10 ± 0.10.