Non-Thermal Leptogenesis: Mechanisms & Models

• Non-thermal leptogenesis is an asymmetry-generation mechanism where heavy neutrinos produced by an out-of-equilibrium parent field decay to yield a lepton asymmetry independent of thermal scatterings.
• It employs detailed Boltzmann equations and modified cosmic expansion to capture the impact of reheating, CP violation, and washout effects on the final baryon asymmetry.
• Various models—including inflaton decay, axion-driven mechanisms, and cosmic-string frameworks—offer testable predictions linking baryogenesis with gravitational waves and collider signals.

Non-thermal leptogenesis denotes the class of baryogenesis mechanisms in which the lepton asymmetry is generated without relying on a thermal population of the heavy states responsible for lepton-number violation. In the canonical decay-based version, heavy right-handed neutrinos are produced predominantly by the decay of an out-of-equilibrium parent field such as the inflaton or a symmetry-breaking scalar, and then decay into lepton and Higgs states with CP violation; in broader realizations, the asymmetry can instead be driven by a time-dependent background field or by a nonstandard expansion history that prevents thermalization of the relevant sector (Zhang, 2023). Across these variants, the central departure from thermal leptogenesis is that the abundance, timing, and momentum distribution of the asymmetry-producing states are inherited from post-inflationary dynamics rather than from equilibrium scatterings, so the final baryon asymmetry retains explicit sensitivity to reheating, parent-field branching ratios, and model-dependent washout suppression (Kusenko et al., 2014).

1. Definition and conceptual scope

In thermal leptogenesis, the heavy Majorana neutrinos $N_i$ are produced by the radiation bath and the standard requirement is $T_{\rm reh} \gtrsim M_N$; the asymmetry then arises from their out-of-equilibrium decays $N \to \ell H$ and $N \to \bar\ell H^\dagger$, followed by sphaleron conversion (Ghoshal et al., 2022). In non-thermal leptogenesis, by contrast, one can have $M_N \gg T_{\rm reh}$, with the heavy sector populated directly by inflaton decay, by the decay of a $B\!-\!L$ breaking scalar, or by another non-equilibrium source, while inverse decays and $\Delta L \neq 0$ scatterings are Boltzmann suppressed (Zhang, 2023).

The literature represented here includes several distinct meanings of “non-thermal.” The most common is direct heavy-particle production from an out-of-equilibrium parent field, as in inflaton decay or $B\!-\!L$-Higgs decay (Goshal et al., 16 Dec 2025). A second meaning covers scenarios in which the heavy neutrinos are never produced on shell and instead appear only virtually in lepton-number-violating scatterings; the axion-oscillation mechanism belongs to this class, where a rolling axionlike field induces an effective chemical potential $\mu_{\rm eff}=\dot a/f_a$ and the asymmetry is generated by $\Delta L=2$ processes mediated by superheavy Majorana neutrinos (Kusenko et al., 2014). A third, broader usage includes nonstandard cosmologies in which the microphysics of the decaying states may still be thermal, but a modified Hubble rate prevents equilibration and makes the leptogenesis dynamics effectively non-thermal (Marco et al., 2022).

A common misconception is that non-thermal leptogenesis is synonymous with low reheating temperature. That statement is model dependent. Inflaton-decay constructions can work with $T_{\rm reh} \gtrsim M_N$0 and even $T_{\rm reh} \gtrsim M_N$1 in the strongly non-thermal regime (Zhang, 2023), whereas axion-driven leptogenesis requires $T_{\rm reh} \gtrsim M_N$2 in order to keep $T_{\rm reh} \gtrsim M_N$3 interactions in equilibrium while the effective chemical potential is active (Kusenko et al., 2014).

2. Dynamical structure and Boltzmann description

The decay-chain version of non-thermal leptogenesis is governed by coupled evolution equations for the parent field, the heavy neutrinos, radiation, and the $T_{\rm reh} \gtrsim M_N$4 asymmetry. In inflaton-based formulations a representative system is

$T_{\rm reh} \gtrsim M_N$5

$T_{\rm reh} \gtrsim M_N$6

$T_{\rm reh} \gtrsim M_N$7

$T_{\rm reh} \gtrsim M_N$8

with $T_{\rm reh} \gtrsim M_N$9 and $N \to \ell H$0 the inflaton partial widths into RHNs and radiation, respectively (Ghoshal et al., 2022). In scalar-source formulations the same logic is expressed in terms of yields. For a non-thermal $N \to \ell H$1 scalar $N \to \ell H$2 decaying to RHNs,

$N \to \ell H$3

$N \to \ell H$4

$N \to \ell H$5

so that a large initial $N \to \ell H$6 acts as a pure non-thermal injection of RH neutrinos (Goshal et al., 16 Dec 2025).

The central parametric quantity controlling washout is the decay parameter

$N \to \ell H$7

or, equivalently, the temperature $N \to \ell H$8 at which RHN decays compete with Hubble expansion (Zhang, 2023). In inverse-seesaw $N \to \ell H$9 models with large Yukawas, the naive washout parameter can be enormous, $N \to \bar\ell H^\dagger$0, but if RHNs are injected at $N \to \bar\ell H^\dagger$1 the effective washout becomes

$N \to \bar\ell H^\dagger$2

which may be moderate even when the underlying Yukawa sector is strongly coupled (Delepine et al., 8 Jan 2026).

The non-thermal RHN yield at reheating is often written as

$N \to \bar\ell H^\dagger$3

for a heavy parent scalar $N \to \bar\ell H^\dagger$4, and the final baryon asymmetry takes the schematic form

$N \to \bar\ell H^\dagger$5

or, in entropy-normalized language, $N \to \bar\ell H^\dagger$6 in the Standard Model (Delepine et al., 8 Jan 2026). In the weak-washout limit of a $N \to \bar\ell H^\dagger$7-scalar source one obtains the simpler estimate

$N \to \bar\ell H^\dagger$8

with $N \to \bar\ell H^\dagger$9 for sufficiently late RHN production (Goshal et al., 16 Dec 2025).

3. Washout, efficiency, and characteristic parameter regimes

A systematic classification of inflaton-decay leptogenesis identifies four characteristic limits in the $M_N \gg T_{\rm reh}$0 plane: RHN dominance, instantaneous reheating, thermalized RHNs, and strongly non-thermal RHNs (Zhang, 2023). RHN dominance and instantaneous reheating occur for small $M_N \gg T_{\rm reh}$1, while the “thermalized RHNs” corner corresponds to large $M_N \gg T_{\rm reh}$2 and $M_N \gg T_{\rm reh}$3, reducing the dynamics to ordinary thermal leptogenesis. The most distinctive regime is “strongly non-thermal RHNs,” in which $M_N \gg T_{\rm reh}$4 but $M_N \gg T_{\rm reh}$5, so that RHNs decay rapidly after production yet never thermalize because the reheating bath never reaches their mass scale (Zhang, 2023).

In that classification, three of the four limits are truly non-thermal, and the strongly non-thermal RHN scenario occupies a large parameter space, including the oscillation-preferred $M_N \gg T_{\rm reh}$6 range, with successful leptogenesis for $M_N \gg T_{\rm reh}$7 and a lower bound $M_N \gg T_{\rm reh}$8 in the relevant large-$M_N \gg T_{\rm reh}$9 region (Zhang, 2023). When two-flavor effects are included, the absolute minimum can be lowered to $B\!-\!L$0 for $B\!-\!L$1 (Zhang, 2023).

Specific models can evade the conventional Davidson–Ibarra scale even more strongly. In the $B\!-\!L$2 plus cosmic-string framework, a hierarchical non-thermal source from $B\!-\!L$3 gives

$B\!-\!L$4

so for $B\!-\!L$5 one finds $B\!-\!L$6, whereas the corresponding thermal bound is $B\!-\!L$7 (Goshal et al., 16 Dec 2025). The physical reason is explicit in the same analysis: late production at $B\!-\!L$8 suppresses inverse decays, so the final asymmetry retains the memory of the injected RHN abundance rather than being driven to the strong-washout attractor (Goshal et al., 16 Dec 2025).

Near-resonant enhancement can reduce the viable scale much further. In the same $B\!-\!L$9 setup, imposing a perturbative but near-resonant condition

$\Delta L \neq 0$0

still allows

$\Delta L \neq 0$1

and numerical solutions show that non-thermal leptogenesis can work down to $\Delta L \neq 0$2 (Goshal et al., 16 Dec 2025). In the $\Delta L \neq 0$3 inverse-seesaw model, the resonant condition $\Delta L \neq 0$4 can be realized with $\Delta L \neq 0$5 for $\Delta L \neq 0$6, yielding $\Delta L \neq 0$7 and successful baryogenesis for $\Delta L \neq 0$8 and $\Delta L \neq 0$9 (Delepine et al., 8 Jan 2026). This does not imply that TeV-scale non-thermal leptogenesis is generic; it is a model-dependent consequence of resonant CP enhancement combined with suppressed washout.

Flavor effects are not a minor correction in these settings. In the $B\!-\!L$0 cosmic-string analysis, flavored Boltzmann equations and fitted non-thermal efficiencies $B\!-\!L$1 enlarge the viable parameter space and can “rescue” regions excluded in unflavored treatments, especially at larger $B\!-\!L$2 (Goshal et al., 16 Dec 2025). In the systematic inflaton-decay study, two-flavor effects lower the minimum allowed $B\!-\!L$3 and modify the large-$B\!-\!L$4 efficiency structure (Zhang, 2023).

4. Principal model realizations

A prominent class of realizations identifies the non-thermal parent with the field that breaks $B\!-\!L$5. In the type-I seesaw model with a complex scalar $B\!-\!L$6, the symmetry-breaking vev $B\!-\!L$7 generates $B\!-\!L$8, while the radial mode $B\!-\!L$9 can decay non-thermally into RHNs and, in the same construction, the symmetry breaking produces cosmic strings whose tension scales as $\mu_{\rm eff}=\dot a/f_a$0 for local strings and acquires an additional logarithm for global strings (Goshal et al., 16 Dec 2025). In the gauged $\mu_{\rm eff}=\dot a/f_a$1 inverse-seesaw model, the heavy $\mu_{\rm eff}=\dot a/f_a$2 Higgs $\mu_{\rm eff}=\dot a/f_a$3 takes over the role usually played by the inflaton: $\mu_{\rm eff}=\dot a/f_a$4 generates the non-thermal RHN population, while the threshold choice $\mu_{\rm eff}=\dot a/f_a$5 suppresses the decay width through the $\mu_{\rm eff}=\dot a/f_a$6 phase-space factor and permits $\mu_{\rm eff}=\dot a/f_a$7 without forcing the relevant Yukawas to be tiny (Delepine et al., 8 Jan 2026).

Several works embed the mechanism directly into inflationary GUT or seesaw constructions. In a TeV-scale $\mu_{\rm eff}=\dot a/f_a$8 inverse-seesaw model, inflaton decay $\mu_{\rm eff}=\dot a/f_a$9 with $\Delta L=2$0, $\Delta L=2$1, and $\Delta L=2$2 yields $\Delta L=2$3 while keeping the heavy neutrinos at the TeV scale (Abdallah et al., 2012). In a 5D orbifold $\Delta L=2$4 model with smooth hybrid inflation, the inflaton $\Delta L=2$5 decays into scalar RH neutrinos, and the baryon asymmetry is obtained as a function of the lightest neutrino mass; the analysis finds that the normal hierarchy can reproduce the observed asymmetry at $\Delta L=2$6, whereas the inverted hierarchy is too small (Fukuyama et al., 2010). In the supersymmetric economical $\Delta L=2$7 model, a loop-induced inflaton coupling to RH neutrinos leads to non-thermal leptogenesis with $\Delta L=2$8 and the relation $\Delta L=2$9 when $T_{\rm reh} \gtrsim M_N$00 (Huong et al., 2010).

Majoron-based inflation provides another unified implementation. In Majoron hilltop inflation the field that breaks lepton number and gives RH-neutrino masses also serves as the inflaton, with superpotential coupling $T_{\rm reh} \gtrsim M_N$01 and hilltop potential

$T_{\rm reh} \gtrsim M_N$02

The heavy sneutrinos are produced both perturbatively and by resonant preheating, and numerical Boltzmann evolution shows that preheating can enhance the baryon asymmetry by more than an order of magnitude (Antusch et al., 2018). In a supersymmetric left-right model with smooth hybrid inflation, the inflaton instead produces $T_{\rm reh} \gtrsim M_N$03 triplets $T_{\rm reh} \gtrsim M_N$04, whose decays to $T_{\rm reh} \gtrsim M_N$05 and $T_{\rm reh} \gtrsim M_N$06 realize non-thermal type-II leptogenesis, while the tiny induced triplet vevs generate light neutrino masses (Khalil et al., 2012).

There are also three-body and dark-sector variants. In the model with a $T_{\rm reh} \gtrsim M_N$07-odd inflaton singlet $T_{\rm reh} \gtrsim M_N$08, minimal dark matter triplet $T_{\rm reh} \gtrsim M_N$09, heavy fermion triplet $T_{\rm reh} \gtrsim M_N$10, and a type-I or type-II seesaw completion, the inflaton decays as $T_{\rm reh} \gtrsim M_N$11, and the CP asymmetry is proportional to $T_{\rm reh} \gtrsim M_N$12 because all physical lepton-sector phases are placed in the type-I/II contribution while the type-III sector is real (Zhou et al., 2016). This suggests a useful organizing distinction between models in which CP violation is inherited from conventional heavy-neutrino decay asymmetries and models in which it is inherited more directly from the low-energy neutrino mass matrix.

5. Alternative non-thermal sources and nonstandard cosmologies

Non-thermal leptogenesis need not proceed through direct on-shell production of heavy neutrinos. In the axion-oscillation mechanism, a light axionlike field with coupling

$T_{\rm reh} \gtrsim M_N$13

induces an effective chemical potential

$T_{\rm reh} \gtrsim M_N$14

and the equilibrium lepton number density becomes

$T_{\rm reh} \gtrsim M_N$15

Here the heavy Majorana neutrinos are never thermally produced; they have $T_{\rm reh} \gtrsim M_N$16 and contribute only through virtual $T_{\rm reh} \gtrsim M_N$17 scatterings with effective cross section

$T_{\rm reh} \gtrsim M_N$18

while successful leptogenesis requires $T_{\rm reh} \gtrsim M_N$19 (Kusenko et al., 2014). This mechanism is formally closer to spontaneous baryogenesis than to decay-generated leptogenesis, but it falls within the broader non-thermal category because the asymmetry originates in a classical background rather than in a thermal heavy-particle population.

A different generalization modifies the background expansion instead of the source term. In brane-inspired multi-scalar cosmologies the Hubble rate is enhanced to

$T_{\rm reh} \gtrsim M_N$20

and the unflavored Boltzmann equations acquire an overall $T_{\rm reh} \gtrsim M_N$21 suppression in the decay and washout terms (Marco et al., 2022). The effect is to replace the usual washout parameter by an effective one $T_{\rm reh} \gtrsim M_N$22, so a regime that would be strong washout in radiation domination can become weak washout in the modified background (Marco et al., 2022). Closely related behavior appears in dark-sector-assisted leptogenesis with a fast-expanding pre-BBN universe, where singlet scalar decays generate the asymmetry and the exponent $T_{\rm reh} \gtrsim M_N$23 in the expansion law yields $T_{\rm reh} \gtrsim M_N$24, whereas the corresponding $T_{\rm reh} \gtrsim M_N$25 case falls well below the observed value (Konar et al., 2020).

The Neutrino Option provides another non-thermal variant with unusually tight structural constraints. In the minimal two-RHN seesaw, the requirement that RH-neutrino loops dynamically generate the Higgs mass limits the heavy-neutrino scale to $T_{\rm reh} \gtrsim M_N$26, rendering standard hierarchical thermal leptogenesis impossible (Samanta et al., 2020). Non-thermal RHN pair production from inflaton decay then becomes the relevant channel, allowing successful baryogenesis for $T_{\rm reh} \gtrsim M_N$27 and $T_{\rm reh} \gtrsim M_N$28 in the pair-production case, with only mild resonance rather than the strong resonance required thermally (Samanta et al., 2020). The same analysis identifies a “phantom window,” $T_{\rm reh} \gtrsim M_N$29 up to $T_{\rm reh} \gtrsim M_N$30, in which the total CP asymmetry decreases as the RH scale increases (Samanta et al., 2020).

This broader body of work suggests that non-thermal leptogenesis is best viewed as a family of out-of-equilibrium asymmetry-generation mechanisms characterized by nonthermal initial conditions, delayed injection, or nonstandard background evolution, rather than by a single inflaton-decay template.

6. Constraints, probes, and current directions

In $T_{\rm reh} \gtrsim M_N$31 constructions, the same symmetry-breaking scale that controls $T_{\rm reh} \gtrsim M_N$32 also sets the cosmic-string tension and hence the stochastic gravitational-wave background, so gravitational-wave data become a direct probe of non-thermal leptogenesis parameter space (Goshal et al., 16 Dec 2025). In that analysis the detectable ranges are $T_{\rm reh} \gtrsim M_N$33 for local strings and $T_{\rm reh} \gtrsim M_N$34 for global strings, while current PTA and CMB measurements already impose upper bounds on the same scale (Goshal et al., 16 Dec 2025). Flavor effects are phenomenologically critical there because they recover regions that would be excluded in unflavored analyses once current CMB or gravitational-wave limits are imposed (Goshal et al., 16 Dec 2025).

High-scale inflaton-decay leptogenesis can also leave indirect signatures in the reheating imprint on CMB observables. In $T_{\rm reh} \gtrsim M_N$35-attractor models, imposing successful non-thermal leptogenesis and the requirement $T_{\rm reh} \gtrsim M_N$36 narrows the allowed $T_{\rm reh} \gtrsim M_N$37 interval substantially; for example, the numerical ranges quoted are $T_{\rm reh} \gtrsim M_N$38 for $T_{\rm reh} \gtrsim M_N$39 and $T_{\rm reh} \gtrsim M_N$40 for $T_{\rm reh} \gtrsim M_N$41 (Ghoshal et al., 2022). The same study finds $T_{\rm reh} \gtrsim M_N$42 when inflaton decays dominantly to radiation and $T_{\rm reh} \gtrsim M_N$43 when inflaton decays dominantly to RHNs (Ghoshal et al., 2022). This does not provide a direct test of leptogenesis, but it does show that reheating-sensitive non-thermal scenarios can be constrained by the same $T_{\rm reh} \gtrsim M_N$44–$T_{\rm reh} \gtrsim M_N$45 data that constrain inflation.

Low-scale realizations are motivated by collider and dark-sector phenomenology. Near-resonant non-thermal leptogenesis in the $T_{\rm reh} \gtrsim M_N$46 string setup remains viable down to the TeV scale, explicitly as a collider target complementary to the gravitational-wave signal (Goshal et al., 16 Dec 2025). In the $T_{\rm reh} \gtrsim M_N$47 inverse-seesaw model the same TeV-scale region contains a $T_{\rm reh} \gtrsim M_N$48, extra Higgs states, and heavy neutrinos with large Yukawas, while the requirement that $T_{\rm reh} \gtrsim M_N$49 remain sizable constrains portal couplings and scalar mixing (Delepine et al., 8 Jan 2026). In the minimal-dark-matter inflaton model the triplet dark matter retains the usual $T_{\rm reh} \gtrsim M_N$50 relic-density prediction and yields a one-loop spin-independent direct-detection cross section $T_{\rm reh} \gtrsim M_N$51 (Zhou et al., 2016). In the Neutrino Option with freeze-in dark matter from inflaton decay, the combined relic-density and free-streaming constraints require $T_{\rm reh} \gtrsim M_N$52 (Samanta et al., 2020).

Three broad directions currently structure the subject. First, systematic classifications of non-thermal regimes now make explicit when the mechanism is genuinely non-thermal and when it merely reproduces thermal leptogenesis after an unconventional production stage (Zhang, 2023). Second, unified models increasingly connect baryogenesis to other observables—gravitational waves, CMB reheating observables, collider signatures, or dark matter—so the relevant parameter space is no longer determined by baryon asymmetry alone (Goshal et al., 16 Dec 2025). Third, flavor, resonance, and preheating effects have proved quantitatively decisive rather than peripheral: flavor can rescue otherwise excluded regions, mild degeneracies can lower the leptogenesis scale by many orders of magnitude, and preheating can enhance the asymmetry by more than an order of magnitude (Antusch et al., 2018). A plausible implication is that future progress will depend less on broad parametric estimates and more on fully coupled treatments of reheating, flavor decoherence, and source-specific nonequilibrium dynamics.