Physical null zones and radiation representation

Abstract

General conditions for reconstructing physical null radiation zones in single photon tree amplitudes are given. The systematic analysis has been carried out using invariant quantities. For arbitrary values of masses and charges these zones are always smaller than in the massless and equal charges case. As an application the radiative W boson decay into heavy quarks is studied. This process turns out be a rather sensitive test of the current quark masses m q( M W 2), as well as of the q qW, q q γ and W +W −γ vertices. This is to the presence of a null line in the photon phase space with a location which strongly depends on m q. A recently proposed radiation representation for single photon tree amplitudes is analyzed. Explicit examples are given for a number of cases including fermion and vector boson lines.