Finite Width Effects and Gauge Invariance in Radiative $W$ Production and Decay
U. Baur, D. Zeppenfeld
TL;DR
It is demonstrated how gauge invariance is restored by including the imaginary part of triangle graphs in addition to resumming the imaginary contributions to the {ital W} vacuum polarization.
Abstract
The naive implementation of finite width effects in processes involving unstable particles can violate gauge invariance. For the example of radiative $W$ production and decay, $q\bar q' \to \ellνγ$, at tree level, it is demonstrated how gauge invariance is restored by including the imaginary part of triangle graphs in addition to resumming the imaginary contributions to the $W$ vacuum polarization. Monte Carlo results are presented for the Fermilab Tevatron.