Revisiting the axial form factor of the nucleon for low Q2 pion electroproduction Original Research Article

An extensive programme of charged pion electroproduction off the proton has been performed at the three-spectrometer facility at the Mainz Microton MAMI. The reaction p(e, e′π+)n was measured at a constant invariant mass of W = 1125 MeV and four-momentum transfers of Q2 = 0.058, 0.117, 0.195, and 0.273 (GeV/c)2. For each Q2 value. measurements at different values of ϵ, the virtual photon polarization parameters, allow for a Rosenbluth separation of longitudinal and transverse cross sections. Preliminary results from new measurements in 2000 and 2002 seem to indicate that the transverse cross section at lower Q2 values is considerably lower than expected from previous experiments.