A. Ereditato

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.

Angular Distributions of Muon Pairs Produced by 194-{GeV}/

We present the angular distributions of muon pairs obtained in a high-statistics experiment using a 194-GeV/c π- impinging on a tungsten target. Our results are based on the analysis of 145,000 events with positive Feynmanx and mass above 4.07 GeV/c2, excluding the ϒ region. Simple first-order QCD relations allow us to determine the ratio of annihilation with hard-gluon emission to the sum of annihilation with hard-gluon emission and hard-gluon Compton scattering, which is found to be about 58% to 75%. We determine the parton square intrinsic transverse momenta to be of the order of 0.6 (GeV/c)2, and about 30% larger in the pion than in the nucleon. At largex1, our data agree with the higher-twist hypothesis, and support the interpretation of the relevant scale parameter as the dimuon square transverse momentum.

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We present the angular distributions of high-mass muon pairs produced in a high-statistics experiment by 140 and 194 GeV/c π− beams impinging on a tungsten target, and by 286 GeV/c π− beam on deuterium and tungsten targets. We find no evidence for a center-of-mass energy dependence or a nuclear dependence of the angular distribution parameters. The two parameters λ and μ are found to be essentially independent of any kinematical variable. In contrast, the parameterv increase with the dimuon transverse momentumPT, at variance with recent perturbative QCD predictions. Our statistics at largex1 are insufficient to substantiate the highertwist prediction.

Negative Pions

We compare the differential cross sections of high-mass muon pair production on deuterium and tungsten by incident negative pions of 140 and 286 GeV. We find an indication of a nuclear effect on the nucleon quark distributions comparable in magnitude to what is observed in muon-iron deep inelastic scattering, whereas the pion-quark distribution is unaffected, compatibly with QCD factorization.

Angular Distributions of Muon Pairs Produced by Negative Pions on Deuterium and Tungsten

Abstract We present the first clear evidence for a nuclear dependence of the transverse momentum distribution of massive muon pairs produced in hadronic interactions. Comparing the cross sections for high-mass dimuon production by incident negative pions (140 and 286 GeV) off tungsten and deuterium, we find that their ratio increases with the dimuon transverse momentum. The resulting difference in the mean transverse momentum squared amounts to 0.15±0.03 (syst.) GeV 2 / c 2 and is shown to be independent of the dimuon mass and longitudinal momentum.

Nuclear effects on the nucleon structure functions in hadronic high-mass dimuon production

Abstract We report on the first observation of neutrino induced production of muon pairs in the electromagnetic field of a nucleus. The data has been obtained using the CHARM II detector exposed to the CERN wideband neutrino and antineutrino beams. A clear signal of 55 ± 16 events is seen in a sample of dimuons of opposite charge without visible recoil at the vertex. The cross section is determined and found to be in agreement with the standard model prediction.

Observation of a Nuclear Dependence of the Transverse Momentum Distribution of Massive Muon Pairs Produced in Hadronic Collisions

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.

First observation of neutrino trident production

Abstract We are reporting on a new determination of sin2ϑw from the ratio of v μ e to v e scattering cross sections. A new detector designed for this purpose was exposed tothe Wide Band Neutrino Beamof the 450 GeV (CERN SPS. An analysis of data taken in 1987 and 1988 is presented based on 762 vμe and 1017 v e events. From the ratio of σ( v μ e ) to σ( v μ e ) we determined sin2ϑw=0.233±0.012 (stat) ± 0.008 (syst) without radiative correction. With radiative correction for mt=mH=100 GeV we find sin2ϑw=0.232±0.012(stat)±0.008(syst).

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

Abstract We are reporting an improved determination of the electroweak mixing angle sin2Θw from the ratio of νμe to ν μ e scattering cross sections. The CHARM II detector was exposed to neutrino and antineutrino wide band beams at the 450 GeV CERN SPS. Including new data collected in 1989 we have obtained 1316 ± 56 νμe and 1453 ± 62 ν μ e events. From the ratio of the visible cross sections we determined sin2Θ0=0.239 ± 0.009(stat) ± 0.007(syst) without radiative corrections and g V e g A e =0.047 ± 0.046 . Combining this last result with recent results on gAe at LEP we obtain gVe= −0.023 ± 0.023.

A new determination of the electroweak mixing angle from muon-neutrino electron scattering

Abstract We report on a measurement of coherent single charged pion production in neutrino-nucleus scattering. The analysis is based on data taken with the CHARM II detector in beams of muon-neutrinos and -antineutrinos. The event numbers amount to N ( μ − π ) = 748 and N ( μ + π ) = 631. Cross sections and their dependence on the neutrino energy are determined. The results are in agreement with the predictions of models based on the PCAC hypothesis.