Sterile Neutrinogenesis in Cosmology

• Sterile neutrinogenesis is a unifying term for diverse mechanisms where sterile neutrinos generate cosmic baryon asymmetry or dark matter relics through decay, oscillations, and PBH evaporation.
• The framework employs neutrino portal interactions, seesaw models, and late-phase transitions to dynamically control active–sterile mixing and shape the thermal history.
• Experimental probes range from neutrino oscillation experiments and CEνNS setups to X-ray searches and gravitational-wave signals from PBH-induced neutrino production.

Sterile neutrinogenesis is a nonuniform term in contemporary neutrino and cosmology literature. In one usage, it denotes mechanisms in which sterile neutrinos participate in generating the cosmic baryon asymmetry, typically through heavy-neutrino decay leptogenesis or through GeV-scale sterile-neutrino oscillations. In another usage, especially in recent dark-matter papers, it denotes the cosmological production of a sterile-neutrino relic population, including production by active–sterile oscillations, self-interaction-driven amplification, or Hawking emission from primordial black holes (PBHs). The literature therefore spans eV-scale oscillation phenomenology, keV–GeV dark-matter production, GeV-scale oscillation baryogenesis, and superheavy-seesaw leptogenesis rather than a single narrowly defined mechanism (Dasgupta et al., 2021, Chen et al., 2023, Chen et al., 23 Jul 2025).

1. Terminology and domain of applicability

The expression is best understood as an umbrella label for sterile-neutrino-driven genesis problems, but the underlying physics depends sharply on the mass scale and on whether the object being generated is a baryon asymmetry or a sterile-neutrino abundance. Review literature on sterile neutrinos treats the matter–antimatter asymmetry under leptogenesis driven by heavier sterile neutrinos, whereas recent PBH papers use “PBH sterile neutrinogenesis” specifically for Hawking production of sterile neutrinos (Dasgupta et al., 2021, Chen et al., 2023).

Usage in the literature	Characteristic scale	Representative realization
Baryogenesis through sterile-neutrino dynamics	GeV to superheavy	ARS oscillation baryogenesis; heavy Majorana decay leptogenesis
Sterile-neutrino relic production	keV to TeV	Oscillation freeze-in, self-interaction amplification, PBH evaporation
Indirect laboratory input on light sterile sectors	eV	Reactor disappearance and CE$\nu$NS neutral-current searches

A recurrent source of confusion is the relation between light sterile neutrinos suggested by short-baseline anomalies and the heavier sterile states used in baryogenesis. The eV-scale sector probed by terrestrial oscillation experiments is not the generic sterile sector of classic high-scale leptogenesis, and even in low-scale constructions its role is model dependent rather than universal (Serebrov et al., 2017, Dasgupta et al., 2021).

2. Microscopic frameworks and mass scales

The minimal field-theoretic backbone is the neutrino portal. In review form, the Yukawa interaction is written as

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$

which after electroweak symmetry breaking induces Dirac masses $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$. Because the sterile fields $N^\beta$ are Standard-Model singlets, Majorana masses are allowed, and the type-I seesaw mass matrix takes the block form

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$

In the seesaw regime $\|M_R\|\gg \|M_D\|$, light masses are of order $\|M_D\|^2/\|M_R\|$, whereas the heavy states remain near $\|M_R\|$ (Dasgupta et al., 2021).

The same literature also develops nonminimal hidden-sector realizations. A particularly important example is the late-phase-transition Dirac portal,

$$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$

with a hidden scalar $\phi$ and hidden chiral sterile neutrinos $$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$0. In this class of models, active–sterile mixing is absent at high temperature because

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$1

and appears only after a hidden-sector phase transition. The parametric mixing estimate is

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$2

This shifts sterile-neutrino phenomenology from ordinary matter suppression to symmetry-forbidden mixing above $$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$3 (Vecchi, 2016).

Across the literature represented here, four mass domains recur. eV-scale sterile states are discussed in short-baseline oscillation phenomenology. keV-scale states are discussed primarily as dark matter. GeV-scale states appear in low-scale baryogenesis through sterile oscillations. Much heavier states, with the review quoting $$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$4 GeV under GUT-motivated Yukawas, belong to standard thermal leptogenesis. Recent PBH-driven relic-production scenarios additionally open a warm-dark-matter window at much larger masses, approximately $$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$5 to $$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$6, because the produced sterile neutrinos are born much hotter than in oscillation-based production (Dasgupta et al., 2021, Chen et al., 2023).

3. Baryogenesis through sterile-neutrino dynamics

In the high-scale Majorana-decay picture, heavy sterile neutrinos decay out of equilibrium and generate a lepton asymmetry which sphalerons partially convert into baryon number. The review writes the relevant decay rate as

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$7

and the CP asymmetry of the lightest state as

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$8

In the hierarchical limit,

$$\mathcal{L}_\text{Yukawa}\supset -y^{\alpha\beta}(i\sigma^2 H^*)L^\alpha N^\beta + h.c.,$$9

The resulting baryon-to-photon ratio is summarized as

$M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$0

with the Standard-Model sphaleron conversion factor $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$1 entering the derivation. This is the canonical sterile-neutrino route to baryogenesis in seesaw models (Dasgupta et al., 2021).

Low-scale sterile-neutrino baryogenesis is of a different character. The Akhmedov–Rubakov–Smirnov mechanism uses two nearly degenerate GeV-scale sterile neutrinos that are produced out of equilibrium, oscillate while relativistic, and generate flavor asymmetries in the active sector before sphaleron freeze-out. A full numerical solution of this hot-oscillation regime evolves a momentum- and helicity-dependent sterile density matrix together with Standard-Model lepton asymmetries and baryon number from $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$2 GeV down to $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$3 GeV (Ghiglieri et al., 2017).

For the benchmark analyzed in detail,

$M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$4

with $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$5, $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$6, and $M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$7, the calculation gives

$M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$8

to be compared with the observed

$M_D^{\alpha\beta}=y^{\alpha\beta}v/\sqrt{2}$9

The same work shows that sterile distributions are far from kinetic equilibrium during the baryogenesis epoch, that infrared momentum modes equilibrate much faster than ultraviolet modes, and that a momentum-dependent treatment can shift the final baryon asymmetry by roughly $N^\beta$0 relative to momentum-averaged benchmarks (Ghiglieri et al., 2017).

Late hidden-sector phase transitions alter the viability of such mechanisms. In the Dirac-portal construction of light sterile neutrinos from a late phase transition, the cosmologically preferred region typically has $N^\beta$1, or even $N^\beta$2. That timing protects against sterile thermalization, but it also postpones active–sterile communication until long after the sphaleron epoch $N^\beta$3 GeV. In that sense, late phase transitions act naturally as washout-avoidance frameworks but generally disfavor sterile-neutrino baryogenesis mechanisms that require active-sector asymmetry generation before electroweak freeze-out (Vecchi, 2016).

4. Dark-matter and relic-population neutrinogenesis

A second major meaning of sterile neutrinogenesis concerns sterile-neutrino relic production rather than baryogenesis. One mechanism starts from a Dodelson–Widrow seed abundance and amplifies it through self-interactions. In the minimal scalar-mediated model,

$N^\beta$4

the sterile abundance first receives an oscillation-generated seed and is then amplified by the resonant transformation process

$N^\beta$5

which proceeds efficiently through on-shell $N^\beta$6. Because the rate is proportional to the already existing sterile abundance, the system can enter an approximately exponential growth regime. Two benchmark points,

$N^\beta$7

and

$N^\beta$8

both reproduce $N^\beta$9 despite mixings far below the standard Dodelson–Widrow line. This mechanism is explicitly about sterile-neutrino dark-matter genesis, not baryogenesis (Bringmann et al., 2022).

PBH sterile neutrinogenesis is conceptually distinct. Here PBHs emit sterile neutrinos directly through Hawking radiation, so the production step is gravitational and independent of the active–sterile mixing angle. The Hawking temperature is

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$0

and the average emitted momentum is

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$1

In the radiation-dominated evaporation regime, the present sterile fraction is approximated by

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$2

Because the particles are produced much hotter than thermal warm dark matter, the viable mass window shifts upward to approximately $$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$3 if PBHs do not dominate before evaporating. In the PBH-dominated case, the same mechanism generally yields only a subdominant hot component or dark radiation rather than all of the dark matter (Chen et al., 2023, Chen et al., 2023).

The multi-population generalization extends this logic further. The same sterile species can be produced simultaneously by PBH evaporation and by oscillations or heavy-particle decays, or by two distinct PBH populations with different masses. In that setting the relic spectrum becomes multi-modal. The 2025 analysis emphasizes that PBH-produced populations are the hotter components, with characteristic scaled momenta $$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$4, compared with colder oscillation- or decay-produced components. This converts sterile neutrinogenesis from a single-channel production problem into a spectral-composition problem (Chen et al., 23 Jul 2025).

5. Cosmological timing, washout, and thermal constraints

The central cosmological issue is whether sterile sectors thermalize, recouple, or remain sufficiently sequestered. In the late-phase-transition framework, the hidden-sector matter potential is estimated as

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$5

and suppression of sterile thermalization at $$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$6 requires

$$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$7

For the benchmark $$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$8, this gives $$\begin{pmatrix} 0 & M_D\ M_D^T & M_R \end{pmatrix}.$$9. The collisional recoupling criterion is

$\|M_R\|\gg \|M_D\|$0

with $\|M_R\|\gg \|M_D\|$1, and for $\|M_R\|\gg \|M_D\|$2 eV and $\|M_R\|\gg \|M_D\|$3 the paper finds recoupling whenever $\|M_R\|\gg \|M_D\|$4. This organizes the parameter space into regions with ordinary thermalization, BBN-safe but collisional neutrinos, and a preferred Region C in which no recoupling occurs until after the CMB (Vecchi, 2016).

The same framework gives a diluted hidden-radiation contribution

$\|M_R\|\gg \|M_D\|$5

and relates late phase transitions to possible topological-defect constraints. The qualitative implication is that delayed symmetry breaking can protect against sterile washout and against $\|M_R\|\gg \|M_D\|$6 excess, but only by moving the onset of active–sterile mixing to very late times (Vecchi, 2016).

A complementary constraint arises even at extremely small mixing. For $\|M_R\|\gg \|M_D\|$7, active–sterile oscillations in the thermal plasma experience a sign-changing matter potential around and below the electroweak crossover. The in-medium mixing angle is

$\|M_R\|\gg \|M_D\|$8

and the scattering-broadened conversion rate is

$\|M_R\|\gg \|M_D\|$9

This produces a resonantly enhanced oscillation-driven freeze-in at $\|M_D\|^2/\|M_R\|$0, strengthening BBN and CMB exclusions down to $\|M_D\|^2/\|M_R\|$1 across the $\|M_D\|^2/\|M_R\|$2 to $\|M_D\|^2/\|M_R\|$3 mass range. The same paper notes that if the sterile neutrino decays predominantly into metastable hidden-sector particles, the mechanism can instead seed a dark-matter sector (Alonso-Álvarez et al., 2022).

Structure formation and self-interactions provide additional filters on viable relic populations. In the scalar-mediated keV dark-matter scenario, the relevant suppression scales are the sound horizon and the free-streaming length, and late-time self-interactions are bounded using

$\|M_D\|^2/\|M_R\|$4

In the PBH case, the hot Hawking spectrum shifts warm-dark-matter viability toward MeV–TeV sterile masses, and a relativistic residual component contributes

$\|M_D\|^2/\|M_R\|$5

rather than cold dark matter (Bringmann et al., 2022, Chen et al., 2023).

6. Experimental and observational status

Laboratory probes of light sterile sectors remain important, but their relevance to sterile neutrinogenesis is indirect unless the same light states participate in a larger sterile sector. The NEUTRINO-4 very-short-baseline reactor experiment exemplifies this distinction. It uses the compact $\|M_D\|^2/\|M_R\|$6 SM-3 reactor with active-zone dimensions $\|M_D\|^2/\|M_R\|$7, a movable $\|M_D\|^2/\|M_R\|$8 Gd-loaded liquid-scintillator detector, and a $\|M_D\|^2/\|M_R\|$9–$\|M_R\|$0 m baseline scan to search for eV-scale disappearance via

$\|M_R\|$1

The paper reports no reliable deviation from $\|M_R\|$2 within its own short-baseline precision, yet a combined reactor-anomaly-motivated fit gives

$\|M_R\|$3

The authors explicitly state that it would be premature to interpret this as an observation of a sterile neutrino, because the preferred region depends on external normalization to the reactor-antineutrino anomaly and because cosmic backgrounds strongly limit the precision (Serebrov et al., 2017).

A cleaner proposed test of sterility itself uses neutral-current coherent elastic neutrino–nucleus scattering. In the CE$\|M_R\|$4NS proposal, a decay-at-rest source and a single detector at $\|M_R\|$5 m and $\|M_R\|$6 m provide sensitivity to active-to-sterile conversion because sterile states do not contribute to the neutral-current signal. For the LSND benchmark $\|M_R\|$7, the study quotes sensitivities of $\|M_R\|$8 for a $\|M_R\|$9 kg Ge detector in the baseline run, $$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$0 for a $$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$1 kg Ar detector, and $$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$2 for a dedicated Ge run. This is a direct probe of eV-scale sterile admixture rather than of baryogenesis or dark-matter production (Anderson et al., 2012).

The observational landscape for relic-population neutrinogenesis is broader. In PBH-driven scenarios, the sterile-neutrino abundance is decoupled from the mixing angle, while decay signals are not. That yields a distinctive phenomenology in which radiative decays with photon energy

$$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$3

can appear independently of the production mechanism, and in PBH-dominated histories can coincide with a stochastic gravitational-wave background. The associated peak frequency is estimated as

$$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$4

with a peak amplitude controlled by the PBH abundance parameter $$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$5. The proposed coincidence of X-ray line searches and PBH-evaporation-induced gravitational waves is the sharpest multimessenger signature of PBH sterile neutrinogenesis (Chen et al., 2023, Chen et al., 2023).

The resulting encyclopedic picture is therefore stratified rather than unitary. At heavy masses, sterile neutrinos furnish established frameworks for leptogenesis through decays or oscillations. At lower masses, they admit a variety of relic-production mechanisms whose outputs are constrained by $$\delta{\cal L} = N^\dagger i\bar\sigma^\mu\partial_\mu N -(y_aNhL+{\rm h.c.}) -(y_sN\phi\nu_s+{\rm h.c.}),$$6, BBN, the CMB, structure formation, and self-interactions. At eV scale, terrestrial oscillation experiments probe only a very specific corner of the sterile sector. Sterile neutrinogenesis, in its full modern usage, denotes this entire set of sterile-neutrino genesis mechanisms and constraints rather than a single canonical theory.