Study of prompt neutrino production in proton‐Cu interactions using BEBC in connection with narrowband ν_e results

Evidence for prompt ν production in proton‐Cu interactions at 400 GeV/c is presented using BEBC as detector. An upper limit on the oscillation probability P(ν_e→ν_τ) of 0.35 at 90% CL is given. The hypothesis that the τ lepton has the electron lepton number is ruled out to 90% CL. The interpretation of ν oscillations from the prompt e^±/μ^± ratio seems to be ruled out from the determination of P(ν_e→ν_e) = 1.04±0.15 obtained from a narrowband beam experiment performed by the same collaboration.