External-emission dominance in pion-proton bremsstrahlung

Abstract

The differential cross section for ${\pi }^{\pm }p\to {\pi }^{\pm }p\gamma$ in the $\Delta$ resonance region can be described in detail by an "external-emission dominance" calculation in which the matrix element is based on the first term in the softphoton expansion of Low. The matrix element is gauge- and Lorentz-invariant and has no free parameters.