ZERO-THRESHOLD EXPANSION OF 2-LOOP SELF-ENERGY DIAGRAMS

An algorithm is constructed to derive a small-momentum expansion for t wo-loop two-point diagrams in all cases where, due to the presence of physical thresholds, there are singularities at zero external momentum . The coefficients of this ''zero-threshold'' expansion are calculated analytically for arbitrary masses. Numerical examples, using diagrams occurring in the Standard Model, illustrate the convergence of the ex pansion below the first non-zero threshold.