Abstract
Two-loop contributions to the electromagnetic form factors are calculated in the kinematic regime close to the fermion-antifermion threshold. The results are presented in an expansion in the velocity of the fermions in the c.m. frame up to next-to-next-to-leading order in . The existence of a new Coulomb singularity logarithmic in , which is closely related to the corrections known from positronium decays, is demonstrated. It is shown that due to this Coulomb singularity relativistic corrections to the nonrelativistic cross section of heavy-fermion–antifermion pair production in annihilation cannot be determined by means of conventional multiloop perturbation theory.
- Received 19 June 1997
DOI:https://doi.org/10.1103/PhysRevD.56.7276
©1997 American Physical Society