in the chargino and neutralino sector already at the one-loop level, see Refs. [@Fowler:2009ay; @Fowler:2010eba; @Bharucha:2012nx].

In the present paper we demonstrate that on-shell wave function normalisation factors, evaluated at the complex poles, are well suited for approximating the full mixing propagators also off-shell. In order to achieve this, the elements of the propagator matrix are expressed in terms of simple Breit–Wigner propagators that are multiplied with wave function normalisation factors. We show for various examples that this approximation works very well in practice. In this context we discuss in detail the uncertainties that are associated with the neglected terms in the approximation and provide estimates of their impact, which we find to be in good agreement with the observed numerical deviations between the full result and the approximation. We furthermore demonstrate that besides very significant technical simplifications our approach also has conceptual advantages.

Concerning technical simplifications, the evaluation of the propagator matrix for a given value of the external momentum squared requires the full momentum dependence of all contributing diagonal and off-diagonal self-energies. In our approximation based on wave function normalisation factors that are evaluated at the complex poles, on the other hand, the only momentum dependence that needs 