High-energy collisions and the breakdown of the usual conditions of unitarity and causality

Abstract

We discuss the questions of unitarity and causality in the ordinary and charge-conserving hadronic bremsstrahlung models, which describe quasielastic collisions of two primary particles with the emission of secondary charged pions. While the ordinary hadronic bremsstrahlung model satisfies the usual conditions of unitarity and causality, the charge-conserving hadronic bremsstrahlung model does not. The newly formulated production unitarity (which takes into account the fact that the secondary pions never occur in the initial state) is satisifed by both models. A new notion of the production causality (which seems to be a natural causality property of hadronic bremsstrahlung models) is found to be formally satisfied by both models. In view of the fact that the charge-conserving hadronic bremsstrahlung model gives a good description of the gross features of particle production whereas the ordinary model does not, we suggest that some causality requirements may in general pose overly stringent conditions on theories describing physical phenomena.