Thursday, August 27, 2009, 12:30 pm — Room 2-160, Bldg. 510

The systematics and reorganization of the perturbative loop expansion in high temperature gauge theories is investigated. In addition to infrared divergences in the bare loop expansion with massless fermions, further evidence for the necessity of a systematic restructuring of perturbation theory comes from the explicit gauge parameter dependence of the fermion damping rate at both one and two-loops. The calculation of the two-loop fermion self-energy in QED shows that the leading terms in the high temperature expansion arise from an explicit hard-soft factorization pattern, in which one of the loop integrals is hard (p ~ T), nested inside a second loop integral which is soft (p ~ eT). There are no hard-hard contributions to the two-loop self-energy at leading order at high T. This hard-soft factorization pattern is necessary for the resummation over multiple loops to coincide with the one-loop self-energy calculated with HTL dressed propagators and vertices, and to yield the complete next-to-leading order correction to the self-energy at scales ~eT, which is both infrared finite and gauge invariant. The gauge invariant effective action corresponding to this resummation and possible extensions of the resummation program to softer processes not captured in the HTL approach will also be discussed.

Hosted by: Gregory Soyez

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