Does thermal leptogenesis in a canonical seesaw rely on initial memory?
Pith reviewed 2026-05-23 00:14 UTC · model grok-4.3
The pith
Asymmetries from heavier right-handed neutrinos survive N1 washout due to flavor misalignment in the seesaw model.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Asymmetries generated by the heavier RHNs (N2 and N3) generally possess components that are misaligned in flavor space with respect to N1, resulting in a partially protected contribution that survives the N1 washout. This memory effect arises even when N1 remains dynamically relevant and cannot be captured within the classical Boltzmann framework.
What carries the argument
Density-matrix equations that include decay, inverse decay, scattering processes and Yukawa-induced flavor projections.
If this is right
- At most one right-handed neutrino can lie in the weak washout regime once neutrino mass and mixing constraints are applied.
- The parameter space divides into four distinct dynamical regimes whose final asymmetries must be calculated separately.
- Including projection effects can extend viable leptogenesis points into the sensitivity range of neutrinoless double beta decay experiments.
- The memory contribution modifies the final B-L asymmetry by an amount that depends on the specific regime.
Where Pith is reading between the lines
- Leptogenesis calculations that rely only on Boltzmann equations may systematically under- or over-estimate the asymmetry in parts of parameter space.
- The same misalignment mechanism could affect other early-universe processes that depend on flavor-specific washout rates.
- Future precision measurements of the baryon asymmetry combined with neutrino data could distinguish the density-matrix regimes from classical ones.
Load-bearing premise
The density-matrix equations with the included decay, inverse decay, scattering and Yukawa flavor projections accurately represent the early-universe dynamics without omitted channels or approximations that would alter the misalignment survival.
What would settle it
A measured baryon asymmetry that matches the density-matrix prediction but deviates from the classical Boltzmann result for a hierarchical seesaw point consistent with neutrino oscillation data.
Figures
read the original abstract
It is a common lore that in thermal leptogenesis within the type-I seesaw framework and a hierarchical spectrum of heavy right-handed neutrinos (RHNs), the CP-violating, out-of-equilibrium decay of the lightest RHN ($N_1$) is the only relevant source of the final $B-L$ asymmetry, since any asymmetry produced by the heavier RHNs is expected to be erased by subsequent $N_1$-mediated washout processes. In this work, we revisit this assumption by solving the density-matrix equations, including decay, inverse decay, and relevant scattering processes, and by fully accounting for flavor-projection effects induced by the Yukawa coupling structure. We show that the asymmetries generated by the heavier RHNs ($N_2$ and $N_3$) generally possess components that are misaligned in flavor space with respect to $N_1$, resulting in a partially protected contribution that survives the $N_1$ washout. Unlike the conventional picture of $N_2$-dominated leptogenesis, this memory effect arises even when $N_1$ remains dynamically relevant and cannot be captured within the classical Boltzmann framework. Furthermore, imposing consistency with low-energy neutrino mass and mixing data, we find that at most one RHN can lie in the weak washout regime, which naturally divides the parameter space into four distinct dynamical regimes. We systematically quantify the memory effect in each regime and demonstrate that it can significantly modify the final $B-L$ asymmetry. We find that including projection effects can indeed extend the viable parameter space into the sensitivity range of neutrinoless double beta decay experiments.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims that in thermal leptogenesis with a hierarchical type-I seesaw, the final B-L asymmetry is not determined solely by N1 decays; instead, flavor-misaligned components of the asymmetry generated by N2 and N3 survive N1 washout when the full density-matrix equations (including decays, inverse decays, scatterings, and Yukawa-induced projectors) are solved. This 'memory effect' persists even when N1 is dynamically relevant and cannot be reproduced in the classical Boltzmann framework. Under neutrino-data constraints the authors divide the parameter space into four regimes (at most one RHN in weak washout) and show that the effect can substantially modify the final asymmetry, extending viable regions into the sensitivity of neutrinoless double-beta-decay experiments.
Significance. If the numerical results hold, the work would revise a long-standing assumption in leptogenesis model building by demonstrating that initial-state memory from heavier RHNs can contribute to the observable asymmetry even in the presence of active N1 washout. This would enlarge the viable parameter space and strengthen the link between high-scale leptogenesis and low-energy observables such as 0νββ, while highlighting the necessity of the density-matrix treatment over Boltzmann equations.
major comments (2)
- [density-matrix equations (as described in the abstract and methods)] The central claim that a misaligned component survives N1 washout rests on the completeness of the density-matrix equations. The manuscript states that decay, inverse decay, and 'relevant scattering processes' plus Yukawa projectors are included, but does not demonstrate that omitted channels (additional 2↔2 scatterings, gauge-mediated flavor mixing, or off-diagonal coherence damping) cannot erase the protected component while N1 remains active. This is load-bearing for the memory-effect claim and for the regime classification.
- [regime classification and numerical results] The division of parameter space into four dynamical regimes and the quantitative statement that the memory effect 'can significantly modify' the final asymmetry rely on numerical solutions whose implementation details, validation against known limits (e.g., strong-washout or N2-dominated cases), and error analysis are not provided. Without these, the regime boundaries and the size of the reported effect cannot be assessed.
minor comments (1)
- Notation for the flavor projectors and the precise definition of the 'misaligned component' should be stated explicitly with equation numbers to allow direct comparison with the Boltzmann limit.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and for highlighting these important points regarding the density-matrix treatment and numerical validation. We address each major comment below.
read point-by-point responses
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Referee: [density-matrix equations (as described in the abstract and methods)] The central claim that a misaligned component survives N1 washout rests on the completeness of the density-matrix equations. The manuscript states that decay, inverse decay, and 'relevant scattering processes' plus Yukawa projectors are included, but does not demonstrate that omitted channels (additional 2↔2 scatterings, gauge-mediated flavor mixing, or off-diagonal coherence damping) cannot erase the protected component while N1 remains active. This is load-bearing for the memory-effect claim and for the regime classification.
Authors: We agree that an explicit justification for the set of included processes would strengthen the presentation. The equations solved are the standard density-matrix formulation used in the leptogenesis literature, with the Yukawa projectors providing the flavor misalignment that protects part of the N2/N3 asymmetry. In the hierarchical limit, additional gauge-mediated flavor mixing and higher-order scatterings are parametrically suppressed relative to the included decay, inverse-decay, and ΔL=1,2 processes at the temperatures where N1 washout operates. Nevertheless, to address the concern directly we will add a short subsection discussing the expected size of the omitted channels and why they do not erase the protected component in the regimes under study. revision: partial
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Referee: [regime classification and numerical results] The division of parameter space into four dynamical regimes and the quantitative statement that the memory effect 'can significantly modify' the final asymmetry rely on numerical solutions whose implementation details, validation against known limits (e.g., strong-washout or N2-dominated cases), and error analysis are not provided. Without these, the regime boundaries and the size of the reported effect cannot be assessed.
Authors: We accept that the numerical implementation requires more documentation. In the revised version we will add an appendix that (i) lists the full set of density-matrix equations and the scattering processes retained, (ii) shows validation against the strong-washout limit (where the final asymmetry becomes independent of initial conditions) and against the N2-dominated analytic approximation, and (iii) reports convergence tests with respect to integration step size and parameter sampling. revision: yes
Circularity Check
No significant circularity; derivation follows from solving stated density-matrix equations
full rationale
The paper obtains the memory effect and regime division by numerically solving the density-matrix equations that incorporate decay, inverse decay, scattering, and Yukawa flavor projections, then applying external neutrino oscillation data constraints. This produces the claimed misalignment survival as an output of the dynamics rather than by redefinition, fitting, or self-citation chain. No load-bearing step reduces to an input by construction, and the framework is self-contained against the stated equations without invoking prior author results as uniqueness theorems.
Axiom & Free-Parameter Ledger
free parameters (1)
- RHN masses and Yukawa couplings
axioms (2)
- domain assumption Type-I seesaw with hierarchical RHN spectrum
- domain assumption Density-matrix equations capture all relevant decay, inverse decay, and scattering processes plus flavor projections
Lean theorems connected to this paper
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
solving the density-matrix equations, including decay, inverse decay, and relevant scattering processes, and by fully accounting for flavor-projection effects induced by the Yukawa coupling structure
-
IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
at most one of the RHNs can be in the weak washout regime... four distinct dynamical regimes
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
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Let us assume all RHNs to be in weak washout regime ( ˜mi < 1.0697 × 10−3 eV ∀ i). Now assuming θ2 = θ3 = π 2 in Eq.18 we get ˜m3 − →0 and | cos θ1|2m2 + | sin θ1|2m3 < 1.0697 × 10−3 eV, | sin θ1|2m2 + | cos θ1|2m3 < 1.0697 × 10−3 eV. (19) From the above inequalities, we get m2 + m3 < 2.1394 × 10−3 eV. (20) Similarly in the case of inverse mass ordering (...
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[2]
Now if we take all the RHNs to be in weak washout regime ( ˜mi < 1.0697 × 10−3 eV ∀ i) and θ2 = θ3 = π 2 , we get ˜m3 − →0 and | cos θ1|2m1 + | sin θ1|2m2 < 1.0697 × 10−3 eV, | sin θ1|2m1 + | cos θ1|2m2 < 1.0697 × 10−3 eV, (21) which gives, m1 + m2 < 2.1394 × 10−3 eV. (22) From Eqs.20 and 22, we see that the inequalities can- not be satisfied by the low-e...
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All RHNs are in strong washout regime In this case, we have taken all the RHNs to be in the strong washout regime and varied the two mass ratios M2/M1 and M3/M1. The top panel of Fig. 3 shows the values of δ (colored points) in the plane of M3/M1 and M2/M1 for a typical values of the washout parameters K1 = 101.5, K 2 = 122.5, K 3 = 166.3. And we have als...
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N1 is in weak washout regime Now, we consider the scenario where N1 is in the weak washout regime. As indicated in Fig. 2, if N1 is in the weak washout regime, then the other two RHNs, N2 and N3, must necessarily be in the strong washout regime. We solve the BEs 14 with typical choice of K1 = 9 .7 × 10−3, K 2 = 85 .1, K 3 = 123 .8 along with M1 = 5.97 ×10...
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N2 is in weak washout regime We now move to the case where N2 is in the weak washout regime, while N1 and N3 remain in the strong washout regime. We consider a typical case by taking K1 = 111.9, K 2 = 5.5 × 10−4, K 3 = 151.2 and the mass of N1 is M1 = 3 .7 × 1012 GeV . Figure 6 (top) shows the variation of the values of δ in the plane of M2/M1 and M3/M1. ...
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discussion (0)
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