1998/09/15 by I. Dadic, Dadic, I.
Physics and Astronomy · #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #hep-ph
paper · pdf · doi:10.48550/arxiv.hep-ph/9809395
10 pages, Revtex, no figures, corrected abstract and misprints, minor changes, Talk given at the 5th International Workshop "Thermal Field Theories and Applications", August 10-14, 1998, Regensburg, Germany
arxiv created 1998/10/09 · arxiv updated 2009/11/30
Long standing problem in out-of-equilibrium thermal field theories are pinching singularities. We find that the expressions suspect of pinching require loop particles to be on the mass shell. This fact, with the help of threshold effect and similar effect due to spin, leads to the elimination of pinching in single self-energy insertion approximation to propagator in all propagators appearing in QED and QCD under very mild restrictions on particle densities. This, together with the cancellation of collinear singularities, allows the extraction of useful physical information contained in the imaginary parts of the two loop diagrams. In some cases of interest (π-ρ interaction, electro-weak interaction, decay of Higgs particle, ...) none of the mentioned mechanisms works and one has to resort to the resummed Schwinger-Dyson series. These cases are more sensitive to the limitations related to the finite time range.