Physical consequences of mass singularities

Abstract

We argue that the perturbation expansion of massless field theories are uniquely and unambiguously defined for properly formed observables. In so doing, we generalize the Kinoshita-Lee-Nauenberg theorem to show that there are no observable "mass-shell anomalies". Paradoxically, in realistic circumstances, massive electrons give rise to non-intuitive, enhanced effects, for example, an unexpected background to the search for right-handed charged currents at HERA.