arxiv: 2604.20581 · v1 · submitted 2026-04-22 · ✦ hep-ph

Recognition: unknown

Flavon assisted low scale leptogenesis

Yan Shao , Zhen-hua Zhao

Authors on Pith no claims yet

Pith reviewed 2026-05-10 00:09 UTC · model grok-4.3

classification ✦ hep-ph

keywords leptogenesisflavonsright-handed neutrinosneutrino mixingbaryon asymmetryTeV scaleflavor symmetryseesaw mechanism

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The pith

Flavon fields from flavor models can serve as the scalar singlet enabling successful leptogenesis at the TeV scale without right-handed neutrino mass degeneracy.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that flavons, already introduced in flavor-symmetry models to generate right-handed neutrino masses through their vacuum expectation values, can double as the scalar singlet S that couples to those neutrinos. This coupling introduces new decay channels N_I to N_J plus S, which supply extra sources of CP violation and remove the usual requirement for nearly degenerate right-handed neutrino masses in low-scale leptogenesis scenarios. In a concrete example realizing the TM1 mixing pattern with only one flavon coupling to two right-handed neutrinos, the model simultaneously fits observed neutrino masses and mixing angles while producing the measured baryon asymmetry within a parameter space allowed by current data. A sympathetic reader would care because the construction ties neutrino flavor structure directly to the origin of the matter-antimatter asymmetry at energies accessible to colliders.

Core claim

In a flavor-symmetry neutrino mass model that realizes the TM1 mixing pattern, a single flavon field playing the role of the scalar singlet S and coupling to only two right-handed neutrinos allows the observed neutrino masses and mixing angles to be reproduced while the observed baryon asymmetry is generated through leptogenesis at the TeV scale, all within a parameter space compatible with experimental constraints.

What carries the argument

The flavon field acting as the scalar singlet S, coupled via S N_I N_J terms (I not equal J), which opens the decay channels N_I to N_J S that supply additional CP violation for low-scale leptogenesis.

If this is right

The observed baryon asymmetry is achievable at the TeV scale without requiring near-degenerate right-handed neutrino masses.
Neutrino masses and the TM1 mixing pattern are reproduced consistently with the same flavon that drives leptogenesis.
The parameter space remains compatible with existing experimental bounds on new scalars and right-handed neutrinos.
Only one flavon coupling to two right-handed neutrinos suffices for both tasks.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The mechanism could be embedded in other flavor-symmetry models to connect neutrino mixing patterns more directly to cosmological observables.
Collider searches for new scalar singlets or right-handed neutrinos at the TeV scale could test the viability of this leptogenesis route.
It reduces reliance on high-scale physics for explaining the matter-antimatter asymmetry by leveraging fields already present for flavor reasons.

Load-bearing premise

The single flavon can simultaneously generate the required right-handed neutrino mass spectrum, supply enough CP violation through the new decay channels, and fit all experimental constraints without extra fine-tuning or internal conflicts.

What would settle it

A full parameter scan in which no region simultaneously reproduces the measured neutrino masses and mixing, produces the observed baryon asymmetry, and evades current constraints would falsify the construction.

Figures

Figures reproduced from arXiv: 2604.20581 by Yan Shao, Zhen-hua Zhao.

**Figure 1.** Figure 1: Flavon S mediated one-loop diagrams that enhance the amount of CP violation in the decay N3 → LαH. where ε v I and ε s I can be generally expressed as ε v 3 = Im (3V23V ∗ 22|y2| 2 + 2V33V ∗ 32|y3| 2 )βα32 8πM3(3|V23| 2|y2| 2 + 2|V33| 2|y3| 2) F v 32,R + Im (3V23V ∗ 22|y2| 2 + 2V33V ∗ 32|y3| 2 )βα∗ 32 8πM3(3|V23| 2|y2| 2 + 2|V33| 2|y3| 2) F v 32,L, ε s 3 = Im (3V23V ∗ 22|y2| 2 + 2V33V ∗ 32|y3| 2 )α22α… view at source ↗

**Figure 2.** Figure 2: Evolutions of the RHN abundances YN2 ,N3 and the baryon asymmetry YB as functions of z for a representative parameter combination (see the main text for details). equations and computing the resulting baryon asymmetry. As an illustration, in [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗

**Figure 3.** Figure 3: For the NO (a) and IO (b) cases of light neutrino masses, the allowed values of [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗

**Figure 4.** Figure 4: Viable parameter space of β versus mS for successful leptogenesis in the NO and IO cases of light neutrino masses. The black points denote the parameter region that reproduces the observed baryon asymmetry, while the colored points correspond to constraints from various experimental benchmarks. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗

read the original abstract

Low-scale leptogenesis scenarios, such as the resonant or ARS leptogenesis, typically require a highly degenerate mass spectrum of right-handed neutrinos (RHNs). This requirement can be circumvented by extending the seesaw framework with a scalar singlet $S$ that couples to RHNs via the $S N^{}_I N^{}_J$ terms (with $I \neq J$), which opens up new decay channels $N^{}_I \to N^{}_J S$ and provides additional sources of CP violation, thereby enabling successful leptogenesis at the TeV scale without the need for mass degeneracy. In this work, we point out that the flavon fields, which are introduced in many flavor-symmetry neutrino mass models to be responsible for the generation of RHN masses through the acquisition of non-zero vacuum expectation values, serve as ideal candidates for the $S$ field. Taking as an example a flavor-symmetry neutrino mass model that naturally realizes the experimentally allowed TM1 mixing pattern and has the attractive features that only one flavon field plays the role of $S$ and that it couples to only two RHNs, we demonstrate that the observed neutrino masses and mixing angles can be consistently reproduced, while the observed baryon asymmetry can be achieved within a parameter space compatible with current experimental constraints.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

A single flavon in this TM1 model can act as the scalar singlet to enable non-degenerate TeV-scale leptogenesis while fitting neutrino data, but the joint parameter space needs close checking for hidden correlations.

read the letter

The paper's core move is to notice that the flavon already present in many flavor-symmetry seesaw models can play the role of the extra scalar S in assisted leptogenesis. In their chosen TM1 setup only one flavon couples to two right-handed neutrinos, its VEV sets the RHN masses, and the new decay channels N_I to N_J S supply the CP violation needed at low scale without forcing degeneracy. That is a clean identification and keeps the field content minimal compared with adding a separate singlet by hand. They show that the same parameters can reproduce the observed neutrino masses and mixing angles and still produce the right baryon asymmetry inside current experimental bounds. That concrete linkage between flavor structure and cosmology is the useful part. The soft spot is the viability of the joint fit. The CP asymmetry from the flavon-mediated loops, the required mass splitting to open the decays, the flavon VEV scale, and the suppression of flavor-violating operators all have to line up at once. The abstract states a viable region exists after scanning, but any strong correlation between those quantities could shrink or eliminate it or force extra tuning that is not obvious from the summary. I would want to see the explicit Boltzmann solutions and the full list of constraints they imposed before accepting the claim at face value. This is aimed at people who already work on flavor models and want a concrete way to connect them to leptogenesis. A reader looking for new testable TeV-scale signatures might find the example worth following up. It is coherent enough on its own terms to deserve a serious referee rather than a desk reject; the calculations can be checked and the parameter space either holds or does not.

Referee Report

2 major / 2 minor

Summary. The paper proposes using flavon fields from flavor-symmetry neutrino mass models as the scalar singlet S in an extended seesaw framework for low-scale leptogenesis. In a specific model realizing the TM1 mixing pattern, a single flavon couples to only two right-handed neutrinos (RHNs), generates their masses via its VEV, and opens new decay channels N_I → N_J S that supply additional CP violation. The authors claim this setup reproduces the observed neutrino masses and mixing angles while achieving the measured baryon asymmetry Y_B at the TeV scale, all within a parameter space compatible with current experimental constraints, without requiring RHN mass degeneracy.

Significance. If the numerical demonstration holds, the work provides a concrete link between flavor symmetry models and viable low-scale leptogenesis, using an economical choice of a single flavon to avoid degeneracy tuning. This could offer new testable signatures at colliders and in flavor-violating processes while connecting neutrino mixing patterns directly to the baryon asymmetry. The approach is attractive for its minimality but its impact depends on the robustness of the joint parameter scan.

major comments (2)

[Numerical results / parameter scan section] The central claim that a viable joint parameter space exists (reproducing neutrino data, yielding the observed Y_B via flavon-mediated CP violation, and satisfying all bounds) is load-bearing but rests on numerical results whose details are not fully transparent. The scan must explicitly demonstrate that the required RHN mass splitting for open N_I → N_J S decays does not conflict with the flavon VEV scale needed for the seesaw and TM1 mixing, or force additional tuning beyond what is stated.
[Leptogenesis calculation section] The CP asymmetry generated by interference between tree-level and loop diagrams involving the flavon in the new decay channels must be derived and shown to be sufficient after solving the Boltzmann equations. Without the explicit expression for the asymmetry parameter (analogous to the standard epsilon in resonant leptogenesis) and the resulting Y_B evolution, it is unclear whether the claimed success holds after including all washout effects and constraints.

minor comments (2)

[Model setup] Notation for the flavon couplings and RHN mass matrix should be clarified early to avoid confusion with standard seesaw parameters.
[Results] A table summarizing the viable parameter ranges (flavon VEV, RHN masses, Yukawas) and the corresponding chi-squared for neutrino fit plus Y_B would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We appreciate the positive assessment of the potential impact and address the major comments point by point below, outlining the revisions we will implement to improve transparency and completeness.

read point-by-point responses

Referee: [Numerical results / parameter scan section] The central claim that a viable joint parameter space exists (reproducing neutrino data, yielding the observed Y_B via flavon-mediated CP violation, and satisfying all bounds) is load-bearing but rests on numerical results whose details are not fully transparent. The scan must explicitly demonstrate that the required RHN mass splitting for open N_I → N_J S decays does not conflict with the flavon VEV scale needed for the seesaw and TM1 mixing, or force additional tuning beyond what is stated.

Authors: We agree that greater detail on the parameter scan is needed for full transparency. In the revised manuscript we will expand the numerical results section to include the explicit parameter ranges scanned, the precise conditions imposed on the RHN mass splitting to ensure the N_I → N_J S channels are kinematically open, and the consistency checks with the flavon VEV required by the seesaw mechanism and TM1 mixing. We will add plots or tabulated examples demonstrating that viable points exist without introducing tuning beyond the model assumptions already stated, while satisfying all experimental bounds and reproducing the observed Y_B. revision: yes
Referee: [Leptogenesis calculation section] The CP asymmetry generated by interference between tree-level and loop diagrams involving the flavon in the new decay channels must be derived and shown to be sufficient after solving the Boltzmann equations. Without the explicit expression for the asymmetry parameter (analogous to the standard epsilon in resonant leptogenesis) and the resulting Y_B evolution, it is unclear whether the claimed success holds after including all washout effects and constraints.

Authors: We acknowledge that the explicit derivation of the CP asymmetry and the Boltzmann evolution were not presented in sufficient detail. In the revision we will derive the CP asymmetry parameter ε for the flavon-mediated decays N_I → N_J S, including the relevant interference terms between tree-level and one-loop diagrams. We will also present the Boltzmann equations governing the evolution of the lepton asymmetry and show the resulting Y_B as a function of temperature, explicitly demonstrating that the observed baryon asymmetry is obtained after all washout effects are included. These additions will appear in the leptogenesis section. revision: yes

Circularity Check

0 steps flagged

Model construction with explicit parameter scan shows viable region for neutrino data plus leptogenesis; derivation self-contained

full rationale

The paper selects a flavor-symmetry model realizing TM1 mixing with a single flavon S coupling to two RHNs, reproduces neutrino masses and angles by fitting the model's parameters to data, then solves the Boltzmann equations for the new decay channels to show that the observed baryon asymmetry lies within the same scanned region that also satisfies experimental bounds. This is standard model-building and numerical demonstration rather than any derivation that reduces to its inputs by construction. No self-definitional equations, no fitted inputs relabeled as predictions, and no load-bearing self-citations that would make the central claim tautological. The result is the existence of a compatible parameter space, which is independently checkable by reproducing the scan and calculations.

Axiom & Free-Parameter Ledger

3 free parameters · 3 axioms · 0 invented entities

The construction rests on the standard type-I seesaw plus flavor symmetry breaking, plus multiple adjustable parameters for masses, couplings, and CP phases that are scanned to match observations.

free parameters (3)

Flavon vacuum expectation value
Determines the scale of RHN masses and must be chosen to allow TeV-scale leptogenesis.
Right-handed neutrino masses and mass splittings
Fitted to produce the correct neutrino spectrum and sufficient CP violation.
Yukawa couplings and scalar couplings to RHNs
Multiple complex parameters scanned to reproduce mixing angles and baryon asymmetry.

axioms (3)

domain assumption Type-I seesaw mechanism generates light neutrino masses
Core framework assumed throughout the model.
domain assumption Flavor symmetry is broken by flavon VEVs to produce observed mixing
Foundation of the TM1 pattern realization.
standard math Standard Boltzmann equations govern the evolution of lepton asymmetry
Used to compute the final baryon asymmetry from CP-violating decays.

pith-pipeline@v0.9.0 · 5519 in / 1547 out tokens · 39309 ms · 2026-05-10T00:09:25.925158+00:00 · methodology

discussion (0)

Reference graph

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