Experimental test of the PCAC hypothesis in the reactions νμp→μ-pπ+ and anti νμp→μ+pπ- in the Δ(1232) region

Data on the reactions νμp→μ-pπ+ and anti νμp→μ+pπ- in the Δ(1232) region are presented and a test of the PCAC hypothesis, using a modified version of the Adler model, is performed. The analysis is based on 1081 events in the neutrino and on 180 events in the antineutrino reaction, obtained in a bubble chamber experiment with BEBC at CERN. The experimental cross-sections for an invariant hadronic mass W<1.4 GeV and an (anti)-neutrino energy EνL>10 GeV are determined to be (0.628±0.059).10-38 cm2 for the neutrino and (0.168±0.023).10-38 cm2 for the antineutrino reaction. The Q2 and W distributions, the density matrix elements of the Δ resonance, and moments of the pion angular distribution are discussed. The data are found to be in good agreement with the Adler model in the Q2 region below 1 GeV2. A maximum likelihood fit for the axial mass mA in the axial-vector from factor yields a value on mA=1.31±0.12 GeV. At low Q2 the data confirm the PCAC hypothesis and the discrepancy, formerly observed between the experimental and theoretical cross-sections for νμp→μ-pπ+ at low momentum transfers (Q2< or approx.0.2 GeV2), is understood as being due to inadequate pion 'off-mass-shell' corrections. (orig.)