Two-loop self-energy diagrams with different masses and the momentum expansion

An algorithm for obtaining the power series in the external momentum of two-loop self-energy diagrams with arbitrary masses of the internal particles is examined. The coefficients of the expansion are represented in terms of vacuum two-loop integrals which are calculated for general values of the masses. By comparison with a numerical calculation of some two-loop diagrams occurring in the Standard Model, it is shown that for values of the external momentum below the threshold the first few terms of the expansion provide good approximations to the complete diagrams. (orig.)