Abstract
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.
Highlights
The observed asymmetry between the number densities of baryonic matter and antimatter in the universe [1] cannot be accounted for in the Standard Model (SM)
We propose that a net lepton or baryon asymmetry can be generated from the interference effect of two sets of treelevel decay or scattering diagrams with the same initial and final states, as long as the following two conditions are satisfied: (i) There is a net nonzero lepton or baryon number between the initial and final states.1 (ii) At least one set of decay or scattering amplitudes is complex such that the squared amplitudes for particles and antiparticles are different, giving rise to a net CP-asymmetry
With the width in the numerator of Eq (15), the asymmetry in this simple model can be viewed as the interference effect of right-handed neutrinos (RHNs)-mediated tree-level diagram in the right panel of Fig. 3 and the oneloop correction to the triplet propagator in the s-channel, where the s-channel process corresponds to the following subprocesses ηη → ΔðÃÞ; ΔðÃÞ → LL
Summary
The observed asymmetry between the number densities of baryonic matter and antimatter in the universe [1] cannot be accounted for in the Standard Model (SM). To obtain a baryon/lepton asymmetry in these 1 → 2 decay scenarios, one must consider the interference between tree- and loop-level diagrams. Similar interference effects between tree and loop-level diagrams have been considered for generating the baryon asymmetry from 2 → 2 scattering [11,12,13,14] or annihilation [15,16,17,18,19,20] processes. We argue that the interference between tree and loop-level diagrams is not the only way to generate a nonzero asymmetry from out-of-equilibrium heavy particle decays/annihilations. We propose a new mechanism where it suffices to consider two sets of interfering diagrams at the tree-level only This can be achieved through tree-level 2 → 2 scattering or 1 → n (with n ≥ 3) decay processes mediated by unstable particles. The thermal cross section relevant for the asymmetry in the narrow-width approximation is given in Appendix C
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