Form Factor of the Photopion Matrix Element at Resonance

The inelastic scattering of electrons in hydrogen leading to pion formation has been examined. Measurements were carried out in which a hydrogen target was bombarded by electrons of energy E₁ W h secondary electrons of energy E₂ being detected by a magnetic analyzer at a fixed angle of 75^∘{}. The ene ies E₁ and E₂ were programed together such that the pions were produced at a constant energy near the peak of the pion-nucleon resonance in the (3/2, 3/2) state; at the same time the momentum transfer to the pion-nucleon system was varied. Special procedures were developed to eliminate contributions from competing processes. Approximately three fourths of the observed cross section corresponds to magnetic-dipole absorption of the incident virtual photon; the momentum transfer dependence can be interpreted in terms of a form factor of the difference between the magnetic moments of the neutron and proton. If the electron-scattering radii are assumed for the proton, then the data appear to require an rms radius of the magnetic moment of the neutron of about 1.1×{}10^-13 cm, based on an exponential model; nucleon recoil corrections are still somewhat uncertain.