Baryogenesis via Dark Matter Oscillations

• Baryogenesis from dark matter oscillations is a mechanism where coherent oscillations between nearly degenerate dark states transfer or generate asymmetry via resonant neutron mixing and freeze-in leptogenesis.
• The approach employs two-state Hamiltonians and density-matrix quantum kinetic equations with finite-temperature resonance to model CP-violating oscillatory dynamics in the early universe.
• Key implications include testable cosmological signatures, collider phenomena, and structure formation constraints that bridge dark matter physics with visible baryon generation.

Searching arXiv for the core literature on baryogenesis from dark matter oscillations and closely related oscillation-based mechanisms. First, I’ll retrieve the 2018 neutron–dark matter oscillation paper and then the later freeze-in oscillation papers that directly realize dark-matter oscillation baryogenesis. Searching arXiv for:

• (Bringmann et al., 2018)
• (Berman et al., 2022)
• (Dong et al., 24 Jul 2025)

Baryogenesis from dark matter oscillations denotes a class of mechanisms in which dark-sector states participate directly in coherent oscillatory dynamics and the resulting nonequilibrium evolution either transfers a pre-existing baryonic asymmetry from the dark sector into visible matter or generates flavored lepton asymmetries that electroweak sphalerons convert into baryon number. The literature contains two especially clear realizations. In one, a primordial asymmetry stored in a GeV-scale dark fermion is partially converted into ordinary baryons through resonant neutron–dark matter oscillations at finite temperature just before big-bang nucleosynthesis (Bringmann et al., 2018). In the other, feebly coupled, nearly degenerate dark-matter fermions are produced by freeze-in, oscillate coherently in flavor space, generate flavored lepton asymmetries through CP-violating interference, and then feed baryon number through sphaleron conversion (Berman et al., 2022, Dong et al., 24 Jul 2025).

1. Conceptual structure

The defining ingredients are near-degenerate dark states, off-diagonal mixing or coherent production of interaction-state superpositions, and an out-of-equilibrium epoch during which oscillation phases can be converted into particle asymmetries. In the baryon-transfer realization, the oscillating system is a two-level $n$-$\chi$ sector, where $\chi$ carries baryon number and mixes with the neutron through a small mass term. In the leptogenesis realization, the oscillating system is a pair of nearly degenerate Majorana dark-matter fermions $\chi_1,\chi_2$, produced out of equilibrium and later rescattering into Standard Model leptons.

Two distinct asymmetry logics therefore occur. The neutron–dark matter mechanism does not create baryon number from zero; it redistributes an initial dark-sector baryon asymmetry into visible neutrons while conserving the total baryon number stored in $n+\chi$. The freeze-in mechanisms instead generate flavored lepton asymmetries first and rely on electroweak sphalerons for baryon production. This suggests that “baryogenesis from dark matter oscillations” is not a single dynamical template but a family of oscillation-assisted low-scale asymmetry mechanisms whose microscopic bookkeeping of $B$, $L$, and $B-L$ differs substantially.

Realization	Oscillating states	Asymmetry route
Resonant baryon transfer	neutron and dark fermion $\chi$	dark baryon asymmetry $\rightarrow$ visible baryons
Freeze-in leptogenesis	$\chi$0	flavored lepton asymmetry $\chi$1 sphalerons $\chi$2 baryons
UV freeze-in oscillation baryogenesis	$\chi$3	flavored asymmetry, washout-induced $\chi$4, sphalerons

2. Oscillation dynamics and kinetic descriptions

The oscillation problem is typically formulated either as a two-state Hamiltonian in a medium or as a density-matrix quantum kinetic system. In the neutron–dark matter case, the low-energy interaction is the mass-mixing term

$\chi$5

with mass difference

$\chi$6

The effective Hamiltonian is

$\chi$7

where $\chi$8 and $\chi$9 are finite-temperature forward-scattering shifts. The relevant in-medium splitting is

$\chi$0

and the resonance condition is $\chi$1. Diagonalization gives

$\chi$2

so maximal mixing occurs at $\chi$3 (Bringmann et al., 2018).

The freeze-in literature uses density matrices because coherent production, oscillation, absorption, and washout all matter simultaneously. In the 2022 freeze-in construction, the mode-by-mode quantum kinetic equation is

$\chi$4

while in the 2025 UV freeze-in extension the momentum-integrated system evolves with the vacuum and thermal Hamiltonians, integrated reaction densities, and flavor asymmetries $\chi$5 (Berman et al., 2022, Dong et al., 24 Jul 2025). In both treatments, coherent phases alone are not enough: scattering, flavor structure, and washout convert flavor asymmetries into conserved or partially conserved charge asymmetries.

A recurring technical point is decoherence. In the neutron case it is caused mainly by neutron–pion scattering, which interrupts coherence on a timescale $\chi$6. In freeze-in leptogenesis, coherence survives because the dark fermions are feebly coupled and never thermalize, so the oscillating density matrix remains genuinely out of equilibrium.

3. Resonant neutron–dark matter oscillations

The 2018 construction develops a concrete example in which dark matter is an elementary Dirac fermion $\chi$7 with mass near the neutron mass and assigned baryon number, so neutron–dark matter mixing does not violate baryon number. The favored regime is $\chi$8 MeV, much smaller than the GeV masses themselves. Vacuum mixing is tiny because $\chi$9, but finite-temperature corrections can drive an in-medium level crossing. The neutron shift is dominated by scattering on thermal pions, while the dark-matter shift in a dark $\chi_1,\chi_2$0 plasma is

$\chi_1,\chi_2$1

with $\chi_1,\chi_2$2. Because $\chi_1,\chi_2$3 becomes negative in the hadronic plasma, the resonance condition can be satisfied at $\chi_1,\chi_2$4–$\chi_1,\chi_2$5 MeV, and the dominant transfer occurs around $\chi_1,\chi_2$6 MeV, just before BBN (Bringmann et al., 2018).

The oscillation probability in a static medium is

$\chi_1,\chi_2$7

A heuristic decoherence treatment averages this over the neutron scattering time and gives

$\chi_1,\chi_2$8

With

$\chi_1,\chi_2$9

the conversion fraction obeys

$n+\chi$0

Since $n+\chi$1 and $n+\chi$2, the desired final fraction is $n+\chi$3, so roughly $n+\chi$4 of the primordial $n+\chi$5 asymmetry must be transferred into neutrons. The paper also gives a density-matrix Boltzmann treatment,

$n+\chi$6

and finds that the exact density-matrix treatment and the heuristic formula agree very well across the baryogenesis-relevant parameter space.

The concrete realization uses a dark $n+\chi$7 gauge sector with $n+\chi$8, a dark photon $n+\chi$9, and a dark Higgs $B$0. A UV completion with heavy colored scalar triplets $B$1 induces the low-energy mixing through a dimension-7 baryon-portal operator, giving

$B$2

with $B$3. A nontrivial consistency issue is early asymmetry leakage between the dark and visible sectors; the model addresses this by introducing a heavy Majorana fermion $B$4 whose equilibrium decays enforce $B$5 and sequester the dark asymmetry at high temperature.

Phenomenologically, the preferred baryogenesis region lies near

$B$6

The paper presents two benchmark points. The joint baryogenesis plus neutron-lifetime-anomaly benchmark is

$B$7

while a simpler baryogenesis-only benchmark is

$B$8

The light-dark-photon realization leads to strong self-interactions and dark acoustic oscillations. For $B$9 and $L$0 keV,

$L$1

rising to $L$2 for $L$3 keV. In the full light-dark-radiation setup, thermal coupling down to $L$4 GeV gives

$L$5

which the paper identifies as a sharp cosmological test.

4. Freeze-in leptogenesis via dark-matter oscillations

The 2022 framework studies a direct realization of freeze-in baryogenesis via dark-matter oscillations. The new fields are two singlet Majorana fermions $L$6 and a charged scalar $L$7, coupled through

$L$8

Because the coupling matrix is not diagonal in the $L$9 space, $B-L$0 decays produce coherent superpositions of the two mass eigenstates. The oscillation parameter is controlled by

$B-L$1

The generated asymmetries are first stored in the anomaly-free flavor combinations

$B-L$2

At leading order they are proportional to

$B-L$3

but summing over $B-L$4 yields zero, so the minimal setup needs flavor-dependent washout at $B-L$5 to generate a baryon asymmetry (Berman et al., 2022).

This leads to three model classes. In the Minimal Model, an exact $B-L$6 symmetry stabilizes the dark matter, and the total baryon asymmetry appears only at $B-L$7, analogously to ARS leptogenesis. In the UVDM Model, a heavier scalar $B-L$8 creates an additional coherent dark-matter population, spoiling the $B-L$9 cancellation and generating baryon asymmetry already at $\chi$0. In the Z2V Model, the exact $\chi$1 is dropped and the new coupling

$\chi$2

allows asymmetry stored in $\chi$3 to feed directly into the Standard Model. If one or two $\chi$4 couplings equilibrate, the baryon asymmetry can again arise effectively at $\chi$5.

The electroweak-era baryon asymmetry is related to the Standard Model asymmetry by

$\chi$6

which reduces to

$\chi$7

in the $\chi$8-preserving models, where $\chi$9. Sphaleron decoupling is taken at

$\rightarrow$0

Quantitatively, the Minimal Model is highly constrained. Full quantum kinetic solutions give viable regions roughly with $\rightarrow$1 TeV, $\rightarrow$2 cm, $\rightarrow$3 keV, and $\rightarrow$4 keV if the light component fraction satisfies $\rightarrow$5. The UVDM Model is much less constrained: viable parameter space exists with $\rightarrow$6 keV, $\rightarrow$7 and $\rightarrow$8 of comparable mass, and $\rightarrow$9 as large as tens of meters. The Z2V Model can enhance the asymmetry by about two orders of magnitude relative to the Minimal Model; maximal mixing is viable for $\chi$00 keV and $\chi$01 TeV, and with aggressive tuning successful baryogenesis can extend to $\chi$02 TeV and $\chi$03 MeV. Structure-formation constraints require the dominant dark component to satisfy $\chi$04 keV, and the collider phenomenology is controlled by electroweak production of $\chi$05, leading to prompt, displaced, or heavy-stable-charged-particle signatures depending on lifetime.

5. Oscillation baryogenesis via ultraviolet dark matter freeze-in

The 2025 UV freeze-in extension shifts the production epoch from on-shell mediator decays to reheating-era scattering mediated by heavy fields integrated out of the spectrum. The dark sector contains two Majorana singlet fermions $\chi$06 and heavy scalars $\chi$07. After integrating out the heavy mediators, the benchmark effective theory is

$\chi$08

Here the $\chi$09-operator preserves SM $\chi$10, whereas the $\chi$11-operator explicitly violates SM $\chi$12. The paper emphasizes that if $\chi$13, then SM $\chi$14 is exactly conserved in the EFT and no total SM $\chi$15 asymmetry can be generated (Dong et al., 24 Jul 2025).

The key novelty is that the oscillation timescale must now match the reheating-era production epoch rather than a mediator threshold. The leading production coefficient scales as

$\chi$16

so UV freeze-in is dominated by the highest available temperatures. The oscillation phase is controlled by

$\chi$17

and the optimal condition is

$\chi$18

If $\chi$19 is too small, oscillations begin after freeze-in has effectively ended; if it is too large, the CP-odd phase averages out already at reheating.

The mechanism is structurally ARS-like. At $\chi$20, dark matter is produced; at $\chi$21, flavored lepton asymmetries appear; and at $\chi$22, flavor-dependent washout converts these into a total $\chi$23 asymmetry. Sphalerons then give

$\chi$24

Successful baryogenesis requires reheating above the electroweak crossover,

$\chi$25

and the viable reheating window is

$\chi$26

The simultaneous dark-matter and baryon solution is narrow because the dark-matter abundance scales as $\chi$27 whereas the total asymmetry scales as $\chi$28. The favorable regime is highly asymmetric between the two dark states: $\chi$29 should be much more strongly coupled to the Standard Model than $\chi$30, $\chi$31 should be extremely light, and $\chi$32 should make up today’s dark matter. Successful solutions occur for dark-matter masses roughly in the $\chi$33 keV to MeV range, with viable $\chi$34 extending up to about $\chi$35 MeV at lower reheating temperatures and $\chi$36 keV in viable regions. A benchmark that fits both dark matter and baryogenesis is

$\chi$37

corresponding to $\chi$38 TeV.

Because the UV-produced spectrum is hotter than a Fermi–Dirac distribution, cosmological probes are central. For $\chi$39, the heavier $\chi$40 must satisfy approximate lower bounds

$\chi$41

If $\chi$42 remains relativistic through recombination, it contributes

$\chi$43

Because $\chi$44 breaks the would-be stabilizing $\chi$45, dark matter is metastable, and X-ray line constraints can be severe unless the EFT flavor structure suppresses the induced radiative decay operators.

6. Scope, misconceptions, and neighboring mechanisms

The term “baryogenesis from dark matter oscillations” is narrower than several neighboring proposals that also link dark matter to the baryon asymmetry. Conversion-driven freeze-out and conversion-driven leptogenesis rely on semi-efficient thermal conversions, CP-violating decays or partial widths, and Boltzmann-equation nonequilibrium, but explicitly not on coherent oscillatory evolution; the relevant dynamics are incoherent conversions between dark matter and a nearly degenerate partner rather than density-matrix precession (Heisig, 2024, Heisig, 10 Apr 2025). Filtered baryogenesis instead uses a first-order phase transition, CP-violating wall interactions, diffusion, and portal-mediated transfer of a dark chiral asymmetry; it is a transport mechanism, not an oscillation mechanism (Baker et al., 2021).

Other oscillation-assisted models also fall outside the strict definition. In baryogenesis from $\chi$46 mesons, the oscillating states are neutral visible-sector mesons, not dark matter, even though dark-sector particles carry the compensating baryon number and provide the relic abundance (Elor et al., 2018, Alonso-Álvarez et al., 2019). In the $\chi$47MSM, baryogenesis is driven by oscillations of the heavier quasi-degenerate sterile pair $\chi$48, whereas the dark matter is the light sterile neutrino $\chi$49; the sterile-neutrino sector is unified, but the oscillating baryogenesis-driving species are not the dark-matter state (Canetti et al., 2012). Dark-matter-induced symmetry breaking of a $\chi$50-charged scalar and the complex-axion construction similarly involve coherent scalar or axion dynamics tied to dark matter, but the baryogenesis source is either a dark-matter-density-driven scalar condensate or a pre-oscillatory slow-roll phase of the future dark-matter field rather than intrinsic dark-matter flavor or particle-antiparticle oscillations (Sakstein et al., 2017, Brandenberger et al., 2020).

Within the strict oscillation category, the field has established two sharply different paradigms. One uses finite-temperature resonance to convert a primordial dark baryon asymmetry into ordinary baryons at $\chi$51 MeV without violating total baryon number. The other uses feebly coupled, nearly degenerate dark fermions whose coherent freeze-in production and subsequent oscillations generate flavored asymmetries before sphaleron freeze-out. A plausible implication is that future progress will continue to hinge on three tests already emphasized by the existing literature: precision cosmology for light or interacting dark sectors, structure-formation constraints for warm or mixed dark matter, and collider or low-energy probes of the mediators that create, measure, or wash out the oscillating dark states.