A measurement of the axial form factor of the nucleon by the p(e,e 'pi(+))n reaction at W=1125 MeV

The reaction p(e,e'pi(+))n was measured at the Mainz Microtron MAMI at an i nvariant mass of W = 1125 MeV and four-momentum transfers of Q(2) = 0.117, 0.195 and 0.273(GeV/c)(2). For each value of Q(2), a Rosenbluth separation of the transverse and longitudinal cross sections was performed. An effecti ve 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 +/- 0.0 44) GeV larger than the axial mass known from neutrino scattering experimen ts. This is consistent with recent calculations in chiral perturbation theo ry. (C) 1999 Elsevier Science B.V. All rights reserved.