Simple Physical Approach to Thermal Cutting Rules

Abstract

A first-principles derivation is given of the imaginary part of a Green's function in real-time thermal field theory. The analysis and its conclusions are simpler than in the usual circled-vertex formalism. The relationship to Cutkosky-like cutting rules is explained.

Comments: 9 pages, plain tex, 5 figures embedded with epsf -- 2 typos corrected