Leptogenesis without on-shell right-handed neutrinos

arXiv:2605.24163 · hep-ph, astro-ph.CO · Submitted 2026-05-22 · Read on arXiv

Listen

Transcript

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.

Simon Cl´erya Alejandro Ibarraa, Onur Yonara

Technical University of Munich

hep-ph, astro-ph.CO

Submitted: 2026-05-22

Updated: 2026-10-05

Comments: 17 pages, 4 figures

Journal ref: JHEP 10 (2026) 038

DOI: 10.1007/JHEP10(2026)038

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 76/100

The gist: 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

Key concepts

Scalar Field $\phi$: A new, lighter scalar field is introduced into the model. This field couples to right-handed neutrinos and decays into lepton doublets and Higgs doublets. Its existence allows for leptogenesis even when the heavy right-handed neutrinos are not present in the early universe.
Off-Shell Right-Handed Neutrinos: These are right-handed neutrinos that are not produced on their mass shell during the process. The scalar field $\phi$ decays into leptons and Higgs doublets by interacting with these off-shell neutrinos, which is a key feature of this mechanism.
CP Asymmetry ($\epsilon_{CP}$): This measures the difference between two decay rates: one favoring leptons and one favoring anti-leptons. This asymmetry arises from the interference between different decay diagrams (tree-level and loop diagrams) in the four-body decay of $\phi$, leading to a net excess of antileptons.
Baryon Asymmetry ($Y_B$): The final goal is to generate an excess of matter over antimatter, which is converted into the observed baryon asymmetry via sphaleron processes. The paper calculates the yield of this asymmetry based on the CP asymmetry generated by the scalar field's decay.

Terminology

Reheating — 34× in this paper · explained in 2 episodes
Boltzmann Equation — 10× in this paper · explained in 2 episodes
Abundance — 6× in this paper · explained in 1 episode
Core — 2× in this paper · explained in 2 episodes
Benchmark — 1× in this paper · explained in 2 episodes
Symmetry Breaking — 1× in this paper · explained in 3 episodes

Summary

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 via off-shell right-handed neutrinos, circumventing the need for these heavy neutrinos to be present in the primeval plasma.

The Core Mechanism

The paper introduces a new scalar field, denoted as ϕ, which is lighter than the right-handed neutrinos (Ni) and couples to them via a Yukawa coupling. This interaction allows ϕ to decay into two lepton doublets and two Higgs doublets through off-shell right-handed neutrinos. The CP asymmetry is derived from the interference between tree-level and loop diagrams in this four-body decay, specifically the process of ϕ → LlHLl′H. This mechanism generates an excess of antileptons over leptons, which eventually leads to a baryon asymmetry via sphaleron processes.

CP Asymmetry Calculation

The CP asymmetry generated in the 1 → 4 decay is defined as:

ϵCP = Γ(ϕ → LHLH) − Γ(ϕ → LH†LH†) / (Γ(ϕ → LHLH) + Γ(ϕ → LH†LH†)) (Equation 2.2).

The interference between the tree-level and one-loop diagrams generates the CP asymmetry, which is expressed in Equation (2.3). When considering the scenario where ϕ couples just to the lightest right-handed neutrino, N1, and assuming a strong hierarchy among masses, this CP asymmetry is found to be: ϵtot ≃ −3 / (320π Mϕ squared / M1) X k != 1. This result can be compared to the one generated in the decay of the lightest right-handed neutrino N → LH,LH† [2, 9], yielding: ϵNil = 1 / (8π X k != 1) f(M2/M2 1).

Cosmological Realizations

The paper examines two distinct cosmological scenarios for the scalar field ϕ. First, it considers the case where ϕ is identified with the inflaton field, which drives reheating. In this context, the asymmetry is generated during reheating and transferred to a baryon asymmetry by sphalerons. The evolution of energy densities during reheating is governed by Boltzmann equations (Equation 3.3), leading to a maximum temperature Tmax: Tmax ≃ 0.7 × Γϕ ρ end / (g⋆!1/4).

Second, the paper analyzes the case where ϕ is a generic scalar field in thermal equilibrium with the Standard Model plasma. The yield of this scalar field is described by a Boltzmann equation (Equation 4.4). The analysis shows that washout processes, such as those from inverse decays (ID) and ∆L = 2 scatterings, can be neglected because the rates are small compared to the Hubble rate at relevant temperatures: Γ∆L=2/H(T = Tmax) ≲ 1.

Parameter Constraints and Results

The study imposes constraints on the model parameters by requiring that the early Universe remains radiation dominated. This leads to a lower limit on the Yukawa coupling y, derived from ensuring that the decays occur before matter-radiation equality: y ≳ 0.16 M1 / (Mϕ 2)(5/3). Furthermore, successful leptogenesis requires a moderate hierarchy between the right-handed neutrino and scalar field masses. The final result for the baryon yield is given by: YB ≃ 0.2 × g(-1/4)∗ ϵ∆Γ 1/2 ϕ M 1 / (Mϕ 3/2), where the CP asymmetry parameter is related to the decay rate as: ϵ∆ ≃ 1.4 × 10 8 × λϕH xFO Mϕ / (M1(5/2) Mϕ 3).

Conclusion

The model demonstrates that successful leptogenesis can occur in type-I seesaw scenarios even when the maximal temperature of the Universe is below the right-handed neutrino mass scale and right-handed neutrinos are never produced on-shell. The required parameter space is controlled by the off-shell suppression factor given by the masses hierarchy (M1/Mϕ) squared, with thermal freeze-out favoring relatively small mass hierarchies.

The gist: A novel mechanism for generating the baryon asymmetry of the Universe proposes a lighter scalar field that decays into leptons and Higgs doublets via off-shell right-handed neutrinos, circumventing the need for these heavy neutrinos to be present in the primeval plasma.

How it works

Improvements for AI systems

As a fastidious and diligent researcher, I have analyzed the provided scientific paper, Leptogenesis without on-shell right-handed neutrinos, by Cl´erya Alejandro Ibarraa et al. The core physics involves generating the baryon asymmetry via leptogenesis driven by a light scalar field that decays into lepton doublets and Higgs doublets through off-shell right-handed neutrino exchange.

Based on the theoretical framework presented, here are specific improvements to AI systems that could be derived from this research:

) AI System Improvements Derived from the Paper:

AI System Improvement:

Generate Novel Particle Physics Scenarios and Constraints (Model Explorer)

Improved Capability: The system can explore vast parameter spaces defined by high-energy physics models (e.g., Seesaw mechanisms, scalar field interactions). It can specifically test if a proposed mechanism (like the one in the paper) is viable under various cosmological conditions (inflationary reheating vs. thermal equilibrium).
Specific Application: Use the derived constraints on mass hierarchies and Yukawa couplings (Equations 4.16–4.19, Figure 4) to quickly identify regions of parameter space that satisfy both the observed baryon asymmetry and the requirement that right-handed neutrinos remain off-shell or heavy relative to the temperature.
AI System Improvement: Predictive Cosmological Evolution Simulator (Reheating Dynamics)
Improved Capability: The system can simulate the non-trivial evolution of energy densities during reheating, specifically tracking how a decaying scalar field (the inflaton, Section 3) affects the radiation bath and the resulting lepton asymmetry generation.
Specific Application: Predict the resulting temperature profiles (Equations 3.8–3.9) and baryon yields (Equation 3.24), allowing researchers to determine if a specific inflationary model or scalar potential can realistically produce the observed baryon-to-photon ratio without violating constraints on the reheating temperature or introducing unobserved on-shell particles.
AI System Improvement: Asymmetric Washout Process Analyzer (Washout Auditor)
Improved Capability: The system can quantitatively assess the dominance of different lepton asymmetry washout processes (ID vs. LL↔H†H† vs. 2→3 processes) as a function of temperature and Hubble rate, using the derived thresholds (Equation 4.10, 4.12).
Specific Application: Given a specific set of input parameters (e.g., inflaton mass, Yukawa couplings), the AI can calculate the relative rates of these washout processes at key epochs (like Reheating Temperature, TRH) and flag scenarios where washout might become significant enough to wash out the generated asymmetry, thus guiding model builders toward safer parameter regimes.
AI System Improvement: Parameter-to-Observable Mapper (Phenomenological Constraint Engine)
Improved Capability: The system can map fundamental model parameters (like the scalar coupling constant and mass ratios) directly onto the observable baryon asymmetry parameter, providing a predictive tool for experimental verification or falsification.
Specific Application: Given a target observed baryon asymmetry, the AI can invert Equation 3.24 to estimate the required CP asymmetry parameter and then determine which combinations of model parameters (like those in Figure 4) are necessary to achieve that result, effectively acting as a reverse-engineering tool for experimentalists.