Comment on applying dispersion relations to amplitudes with infrared singularities

Abstract

Dispersion Relations (DR) are known to be a powerful instrument for studying scattering amplitudes. In particular, they often apply to calculations in Perturbative QCD and Standard Model. We argue that applying DR to amplitudes with double-logarithmic (DL) contributions should be done with a proper caution because DL terms are often infrared-divergent. Ignoring this circumstance leads to incorrect results. As an example of such situation, we consider applying DR to decays of on-shell W and Z -bosons into fermion pairs in the Double-Logarithmic Approximation