Rehovot 76100, Israel*\
    *(b) DESY, Deutsches Elektronen-Synchrotron, Notkestr. 85,*\
    *D-22607 Hamburg, Germany*
bibliography:
- 'mthesis\_literature.bib'
- 'gNWALoop.bib'
- 'PhDthesis.bib'
- 'bbH.bib'
- 'propagator.bib'
title: '\'
---

Introduction {#chap:intro}
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As a consequence of electroweak symmetry breaking, particles with the same quantum numbers of the electric charge and colour can mix with each other. This is reflected in their propagators contributing to production or decay processes in particle physics. Beyond lowest order those propagators receive contributions from all possible mixings with other particles, such that the propagators of the system of particles that can mix with each other are of matrix-type. Since the involved particles are in general unstable, a proper treatment of imaginary parts is necessary. The mass of an unstable particle is determined from the real part of the (gauge-invariant) complex pole [@Willenbrock:1991hu; @Sirlin:1991fd; @Stuart:1991xk; @Veltman:1992tm], while the imaginary part yields the total decay width of the particle. The eigenstates associated with the masses obtained from the complex poles of the propagator matrix in general differ from the interaction eigenstates that contribute at lowest order.

As an example, in the Standard Model (SM) the neutral interaction eigenstates arising from the ${\rm SU}(2)_{\rm I}$ and ${\rm U}(1)_{\rm Y}$ gauge groups 