Two - Loop Corrections to the Electromagnetic
نویسنده
چکیده
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 O(2 ln) corrections known from positronium decays, is demonstrated. It is shown that due to this Coulomb singularity O(2) relativistic corrections to the non-relativistic cross section of heavy fermion-antifermion pair production in e + e ? annihilation cannot be determined by means of conventional multi-loop perturbation theory.
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