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 $E_1$ W h secondary electrons of energy $E_2$ being detected by a magnetic analyzer at a fixed angle of 75°. The ene ies $E_1$ and $E_2$ were programed together such that the pions were produced at a constant energy near the peak of the pion-nucleon resonance in the ($\frac{3}{2}, \frac{3}{2}$) 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×${10}^{-13\mathrm{}}$ cm, based on an exponential model; nucleon recoil corrections are still somewhat uncertain.