A measurement of the axial form factor of the nucleon by the p(e,e′π+)n reaction at

Abstract The reaction p(e,e′π+)n was measured at the Mainz Microtron MAMI at an invariant mass of W=1125 MeV and four-momentum transfers of Q2=0.117, 0.195 and 0.273 ( GeV/c ) 2 . For each value of Q2, a Rosenbluth separation of the transverse and longitudinal cross sections was performed. An effective Lagrangian model was used to extract the `axial mass' from experimental data. We find a value of M A =(1.077±0.039) GeV which is (0.051±0.044) GeV larger than the axial mass known from neutrino scattering experiments. This is consistent with recent calculations in chiral perturbation theory.