Single π+ production in charged current neutrino-hydrogen interactions

Abstract The reaction ν p→ μ − p π + (777 events) is studied at neutrino energies from 5 to 200 GeV. A total of 551 events fit the channel ν p→ μ − Δ ++ (1232) with χ 2 probability greater than 2%. Decay angular distributions of the Δ ++ are investigated and the density matrix elements are studied as functions of Q 2 , the square four-momentum transfer. The total Δ ++ production cross section and the differential cross section d σ /d Q 2 are determined. Using the Schreiner-von Hippel parametrization of the Adler model, the value of the axial mass determined by fitting the total cross sections above 20 GeV is M A = 0.85±0.10 GeV. The model describes the differential cross sections reasonably well above Q 2 ⋍0.2 GeV 2 . Disagreements, however, are found for some values of the density matrix elements of the Δ ++ decay angular distributions and their Q 2 dependence. The (p π + ) mass region above the Δ ++ (1232) contains evidence for the presence of higher mass Δ ++ resonances and, above 2 GeV, for the forward emission of fast π + .