Leptogenesis without on-shell right-handed neutrinos

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Leptogenesis without on-shell right-handed neutrinos".

Jocelyn: Leptogenesis without on-shell right-handed neutrinos proposes a novel mechanism for generating the baryon asymmetry of the Universe by introducing a lighter scalar field that decays into leptons and Higgs doublets…

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So, to recap where we are, we've talked about how this paper tackles leptogenesis by proposing a mechanism that bypasses the need for right-handed neutrinos to be produced on-shell in the early universe. The core of "Leptogenesis without on-shell right-handed neutrinos" is the introduction of a lighter scalar field, phi.

Jocelyn: That sounds like it's aiming to solve a known problem in standard leptogenesis models by finding an alternative way to generate the necessary CP asymmetry. What exactly is the central claim they are making about this new scalar field?

Subrahmanyan: The authors introduce phi, which is lighter than those heavy right-handed neutrinos, and it couples to them via a Yukawa coupling. This allows phi to decay into leptons and Higgs doublets through those off-shell right-handed neutrinos, which is the key step in generating the CP asymmetry.

Vera: So, instead of relying on direct production of those heavy neutrinos as the starting point, they propose that phi mediates the process indirectly through these off-shell interactions to create a lepton excess. This avoids placing strong constraints on how those heavy neutrinos are created in the primeval plasma.

Jocelyn: That seems like a significant theoretical shift; it changes where the source of CP violation comes from in this context, moving it from direct heavy neutrino decay to an interference effect within a four-body decay process involving phi.

Subrahmanyan: Exactly. The CP asymmetry they derive is generated by the interference between tree-level and loop diagrams in that specific four-body decay, which they define mathematically as epsilon CP = (phi to LHLH) - (phi to LH LH) / ((phi to LHLH) + (phi to LH LH)) (Equation two point two) <ref:2605.24163#pg1>.

Vera: And they show that this calculated asymmetry can successfully reproduce the observed baryon asymmetry both when phi is responsible for reheating the universe and also when phi is just a generic scalar field in thermal equilibrium with the Standard Model plasma. That flexibility makes it a very robust model proposal.

Jocelyn: So, even if we don't have direct evidence of these heavy right-handed neutrinos being produced on-shell, this mechanism still manages to generate the observed matter excess via sphaleron processes. What does this imply for how we look at the history of our universe?

Subrahmanyan: It implies that the generation of the baryon asymmetry doesn't strictly require a specific initial condition for those heavy neutrinos in the very early stages, as long as they are heavier than the temperature of that era. The mechanism is driven by interactions happening at different scales and temperatures, not just one single production event.

Vera: It means we have a viable pathway to generating the cosmic matter-antimatter imbalance that doesn't require those heavy neutrinos to be in a very specific, easily accessible state in the primeval plasma. That's a major win for model building.

Jocelyn: It certainly opens up new avenues for theoretical exploration, pushing us beyond the simplest realizations of leptogenesis we’ve studied so far. It suggests that complexity isn't always required to find a working solution to this cosmological puzzle.

Conclusion: Vera: We've covered a lot about this paper, focusing on how this work tackles leptogenesis by proposing a mechanism that bypasses the need for right-handed neutrinos to be produced on-shell, which is the main thrust of "Leptogenesis without on-shell right-handed neutrinos." The authors are Cl´erya, Ibarraa, and Yonara from TUM.

Jocelyn: And thinking about the title itself, it really captures the essence of what they’ve done: finding a way to generate that crucial cosmic asymmetry without relying on those heavy right-handed neutrinos being present in their most accessible state. What is the biggest implication of this finding for us as observers?

Subrahmanyan: The main implication is that we have found a mechanism where leptogenesis can happen even when the maximal temperature of the Universe is below the mass scale of those right-handed neutrinos, and they are never produced on-shell by any known thermal or non-thermal process.

Vera: In simpler terms, it means that the conditions for generating the matter excess don't have to be met by those heavy particles being readily available in a specific state at every moment in the very early universe. The model is more resilient to certain production constraints.

Jocelyn: That resilience is important because it broadens the landscape of viable cosmological models we can consider when trying to explain why our Universe has so much more matter than antimatter. It gives theorists more freedom.

Subrahmanyan: It certainly does, and the parameter space they constrain—especially concerning the mass hierarchy between phi and M one —is very revealing about what kind of physics must be operating at those intermediate energy scales to make this work <ref:2605.24163#pg1>.

Vera: It’s a great piece of work because it takes a well-known mechanism and shows how it can be adapted to fit tighter constraints on the early universe dynamics, which is exactly what observational cosmology demands.

Jocelyn: So, as we wrap up our discussion on "Leptogenesis without on-shell right-handed neutrinos," we’ve seen how this scalar field phi provides a novel route to generating asymmetry by using off-shell interactions instead of direct production of heavy neutrinos.

Subrahmanyan: It’s a compelling theoretical result that suggests the physics driving baryogenesis can be more subtle and less dependent on specific high-energy production mechanisms than we might initially assume.

Vera: Indeed, this paper provides a solid foundation for exploring alternative pathways to explaining the matter dominance we see today, offering new constraints on particle physics parameters.