Differential-Cross-Section Dip in Quantum Electrodynamics

Abstract

In pair production, wide-angle bremsstrahlung, and trident production, a deep, sharp dip is seen in calculated differential cross sections when the sum of outgoing momenta (excluding that of the target nucleus) is collinear with the incident momentum. It is argued that the dip follows very simply from conservation of the electromagnetic current, for all electromagnetic production processes involving spinless bosons, spin-½ fermions, and/or photons. Sufficient conditions for existence of the dip are discussed.