  We propose a new mechanism for generating matter-antimatter asymmetry via the
interference of tree-level diagrams only, where the imaginary part of the
Breit-Wigner propagator for an unstable mediator plays a crucial role. We first
derive a general result that a nonzero $CP$-asymmetry can be generated via at
least two sets of interfering tree-level diagrams involving either
$2\rightarrow2$ or $1\rightarrow n$ (with $n\geq3$) processes. We illustrate
this point in a simple TeV-scale extension of the Standard Model with an inert
Higgs doublet and right-handed neutrinos, along with an electroweak-triplet
scalar field, where small Majorana neutrino masses are generated via a
combination of radiative type-I and tree-level type-II seesaw mechanisms. The
imaginary part needed for the required $CP$-asymmetry comes from the trilinear
coupling of the inert doublet with the triplet scalar, along with the width of
the triplet scalar mediator. The real part of the neutral component of the
inert doublet serves as a cold dark matter candidate. The evolutions of the
dark matter relic density and the baryon asymmetry are intimately related in
this scenario.
