Prompt neutrino production in 400 GeV proton-copper interactions

A new beam-dump experiment has been performed at the CERN Super Proton Synchrotron using the CHARM neutrino detector. The instrumentation and the statistics have been significantly improved with respect to earlier experiments. For a neutrino energy above 20 GeV the asymmetry of the prompt muon-neutrino and electron-neutrino fluxes \((v_\mu + \bar v_\mu ) - (v_e + \bar v_e )]/(v_\mu + \bar v_\mu ) + (v_e + \bar v_e )]\) is found to be 0.20±0.10 (stat.)±0.05 (syst.), and the asymmetry of prompt antineutrino and neutrino fluxes for muonneutrinos \((v_\mu - \bar v_\mu )/(v_\mu + \bar v_\mu )\) is 0.02±0.16 (stat.)±0.02 (syst.) in agreement with our previous results. For the cross-section times branching ratio for charm production and semileptonic decay we obtain a value of \(\sigma \times BR\left {D(\bar D) \to v_e (\bar v_e ) X} \right] = 1.9 \pm 0.2 \pm 0.2\mu b\) per nucleon. We find no evidence forv _τ orv _x interactions. The \((v_\tau + \bar v_\tau )\) flux is less than 21% of the total prompt neutrino flux. We derive an improved limit on the branching ratio \(\pi ^0 \to v\bar v\) of 6.5×10^?6, and as a verification of the universality of the neutral weak coupling we find \(g_{v_e \bar v_e } /g_{v_\mu \bar v_\mu } = 1.05_{ - 0.18}^{ + 0.15} \) .