  We show how the two physically-distinct sources of CP-asymmetry relevant to
scenarios of leptogenesis: (i) resonant mixing and (ii) oscillations between
different flavours can be unambiguously identified within the Kadanoff-Baym
formalism. These contributions are isolated by analyzing the spectral structure
of the non-equilibrium propagators without relying on the definition of
particle number densities. The mixing source is associated with the usual mass
shells, whereas the oscillation source is identified with a third intermediate
shell. In addition, we identify terms lying on the oscillation shell that can
be interpreted as the destructive interference between mixing and oscillation.
We confirm that identical shell structure is obtained in both the Heisenberg-
and interaction-picture realizations of the Kadanoff-Baym formalism. In so
doing, we illustrate the self-consistency and complementarity of these two
approaches. The interaction-picture approach in particular has the advantage
that it may be used to analyze all forms of mass spectra from quasi-degenerate
through to hierarchical.
