Fermionic Asymmetric Dark Matter

• Fermionic ADM is defined by a primordial dark asymmetry in a non-self-conjugate fermion, often predicting a benchmark GeV-scale mass when visible and dark asymmetries are comparable.
• Models span from minimal singlet Dirac fermions with light scalar mediators to composite twin baryons and keV-scale warm candidates, offering diverse microscopic realizations.
• Efficient depletion of the symmetric component via light mediators and large annihilation cross sections is crucial to distinguish ADM signatures from conventional thermal relic dark matter.

Searching arXiv for the supplied and closely related fermionic ADM papers to ground the article and citations. Fermionic asymmetric dark matter (ADM) denotes dark matter whose present abundance is set by a particle–antiparticle asymmetry, analogous to the baryon asymmetry, with the dark state itself being a fermion. In the standard ADM logic, the abundance relation $$\Omega_{\rm DM}/\Omega_b = m_{\rm DM} n_X/(m_p n_B)$$ implies a mass of order a few GeV if visible and dark asymmetries are comparable, but the fermionic ADM literature also contains composite baryonic realizations, heavier models in which the asymmetry densities are not comparable, and keV asymmetric warm dark matter in which the asymmetry mainly changes the phase-space distribution rather than fixing a GeV-scale mass (Blennow et al., 2012, García et al., 2015, Yin et al., 2024).

1. Relic asymmetry and fermionic quantum numbers

In ADM, the relic abundance is not primarily set by thermal freeze-out of a symmetric particle–antiparticle population, but by a primordial asymmetry in the dark sector. For fermions, that asymmetry is naturally defined when the dark matter is non-self-conjugate. A recurring statement across the literature is that a Majorana fermion cannot carry a conserved particle–antiparticle asymmetry in the usual sense, whereas a Dirac fermion, or another complex fermionic state carrying a conserved dark quantum number, can do so (Blennow et al., 2012). This is why many fermionic ADM constructions are explicitly Dirac, or effectively Dirac before late-time symmetry breaking.

The standard abundance estimate is model-independent. If the mechanism linking the dark and baryon asymmetries conserves a combination of $B-L$ and a dark charge $X$, then $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$ is the characteristic scale, and in one explicit Dirac-type leptogenesis framework the fully asymmetric limit gives $m_\chi \simeq 1.8~\mathrm{GeV}$ through $n_\chi = \frac{79}{28} n_B$ and $\Omega_{\rm DM}/\Omega_B \simeq 5$ (Blennow et al., 2012, Ishida et al., 15 Oct 2025). This does not fix all fermionic ADM models to the GeV scale. A plausible implication is that the GeV prediction is a common benchmark rather than a universal theorem, because heavier ADM can arise if the dark asymmetry is diluted or the dark number density is reduced relative to the baryon asymmetry, and keV asymmetric warm dark matter can use the asymmetry mainly to realize a degenerate Fermi gas (Zhao et al., 2014, Yin et al., 2024).

The distinction from bosonic ADM is structural. In fermionic ADM, compact-star support is governed by Fermi degeneracy pressure rather than Bose condensation, and this difference propagates into neutron-star accumulation, collapse criteria, and compact-star equilibria (Goldman et al., 2013, Gresham et al., 2018). One notable variant even starts with asymmetric Dirac dark matter and converts it into a Majorana fermion only after annihilations have frozen out, so the relic abundance is fixed while the state is Dirac, but the present-day particle avoids the dangerous vector $Z$-exchange characteristic of a Dirac electroweak fermion (Okada et al., 2012).

2. Microscopic realizations and mass generation

The fermionic ADM literature spans elementary singlets, mirror-baryon-like fluids, composite twin baryons, and sterile-neutrino-like warm dark matter. The following examples illustrate the range of constructions.

Framework	Fermionic candidate	Characteristic feature
Minimal light fermionic ADM (Bhattacherjee et al., 2013)	singlet Dirac fermion $\chi$	light real scalar mediator mixed with the Higgs
Fraternal Twin Higgs ADM (García et al., 2015)	spin-$3/2$ twin baryon $B-L$0	$B-L$1
Dirac-type leptogenesis with exact $B-L$2 (Ishida et al., 15 Oct 2025)	Dirac fermion $B-L$3	$B-L$4, with $B-L$5 in the fully asymmetric limit
Asymmetric warm dark matter (Yin et al., 2024)	RHN-like Dirac fermion $B-L$6	$B-L$7 gives a degenerate Fermi gas

In the minimal singlet construction, the Lagrangian is

$B-L$8

with a light real singlet scalar $B-L$9 mixed with the Higgs, so that the physical scalar eigenstates are

$X$0

This is a minimal renormalizable realization of GeV-scale fermionic ADM in which the additional light scalar is introduced because a singlet fermion has no renormalizable coupling to Standard Model fields and the dimension-5 Higgs portal $X$1 is too weak to remove the symmetric component if $X$2 (Bhattacherjee et al., 2013).

Composite fermionic ADM appears naturally in the Fraternal Twin Higgs. There the central candidate is the twin baryon $X$3, a spin-$X$4 fermion whose mass is dynamically set by confinement, $X$5, in a sector where $X$6 is already restricted by Twin Higgs naturalness (García et al., 2015). This is technically important because the ADM mass is not inserted by hand; it is set by a QCD-like scale.

A distinct class ties fermionic ADM to neutrino mass generation and baryogenesis. In the unified Dirac-neutrino model, an exact global lepton number $X$7, a gauged $X$8, and an unbroken $X$9 stabilize a Dirac fermion $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$0, with

$m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$1

Heavy Dirac neutrino decays then generate equal and opposite asymmetries in the visible and dark sectors, with $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$2 (Ishida et al., 15 Oct 2025). This makes the fermionic ADM asymmetry a literal lepton asymmetry stored in the dark sector.

At much lower mass, asymmetric warm dark matter uses a RHN-like Dirac fermion $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$3 with distribution

$m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$4

For $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$5 and $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$6, the anti-DM population dominates, $m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$7, and the system becomes a degenerate Fermi gas (Yin et al., 2024). This suggests that the fermionic ADM label covers both abundance-based and phase-space-based uses of asymmetry.

3. Symmetric-component depletion and hidden-sector interactions

A defining technical requirement in fermionic ADM is efficient removal of the thermally produced symmetric component. In the minimal singlet-scalar model, the dominant process is

$m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$8

with

$m_{\rm DM}\sim (5-7)/Q_{\rm DM}\ {\rm GeV}$9

and thermal average

$m_\chi \simeq 1.8~\mathrm{GeV}$0

Imposing $m_\chi \simeq 1.8~\mathrm{GeV}$1 gives $m_\chi \simeq 1.8~\mathrm{GeV}$2 for $m_\chi \simeq 1.8~\mathrm{GeV}$3 and $m_\chi \simeq 1.8~\mathrm{GeV}$4 for $m_\chi \simeq 1.8~\mathrm{GeV}$5 (Bhattacherjee et al., 2013). These lower bounds are not optional model details; they are the condition that keeps the relic abundance genuinely asymmetric.

A broader model-independent analysis emphasizes the same point. Successful ADM models need a light dark matter candidate, typically near a few GeV, and large annihilation cross sections to eliminate the thermally produced symmetric population. One common way to realize this is a lighter mediator with $m_\chi \simeq 1.8~\mathrm{GeV}$6, into which the dark matter annihilates efficiently; if that mediator decays into lighter dark-sector states rather than Standard Model particles, the hidden sector contributes dark radiation and potentially DM–DR scattering signatures in $m_\chi \simeq 1.8~\mathrm{GeV}$7 and the matter power spectrum (Blennow et al., 2012). The same framework gives

$m_\chi \simeq 1.8~\mathrm{GeV}$8

and parameterizes DM–DR momentum transfer as either

$m_\chi \simeq 1.8~\mathrm{GeV}$9

Specific fermionic models realize this hidden-sector annihilation in different ways. In the Dirac-type leptogenesis model, the dominant annihilation channel is

$n_\chi = \frac{79}{28} n_B$0

mediated by $n_\chi = \frac{79}{28} n_B$1-channel $n_\chi = \frac{79}{28} n_B$2 exchange, and for $n_\chi = \frac{79}{28} n_B$3, $n_\chi = \frac{79}{28} n_B$4, and $n_\chi = \frac{79}{28} n_B$5, a fully asymmetric regime is achieved once $n_\chi = \frac{79}{28} n_B$6 (Ishida et al., 15 Oct 2025). In the asymmetric warm dark matter model, the asymmetry rather than annihilation is the dominant structural ingredient, but the same tiny Yukawa interaction that mixes the sterile fermion with active neutrinos also leaks a small amount of the dark asymmetry into the visible sector, providing baryogenesis by transfer rather than by freeze-out (Yin et al., 2024).

A frequent source of confusion is the assumption that ADM must annihilate into Standard Model states. The EFT reappraisal of Dirac-fermion ADM coupled directly to quarks or leptons shows why this is problematic: the annihilation required to erase the symmetric component imposes an upper bound on the EFT scale $n_\chi = \frac{79}{28} n_B$7, while collider and direct-detection null results impose lower bounds. In that heavy-mediator EFT, quark-coupled ADM in the $n_\chi = \frac{79}{28} n_B$8 range is strongly constrained, whereas leptophilic ADM remains allowed for $n_\chi = \frac{79}{28} n_B$9 (Roy et al., 2024). This suggests that hidden-sector annihilation channels are not an aesthetic addition but often the mechanism that preserves fermionic ADM viability.

4. Compact stars and fermionic ADM equations of state

Fermionic ADM enters compact-star physics through degenerate pressure and, in many models, through self-interactions encoded in a dark-sector equation of state. The generic scaling emphasized in both heuristic and GR treatments is

$\Omega_{\rm DM}/\Omega_B \simeq 5$0

so lighter fermions can support more gravitating mass before relativistic instability sets in (Goldman et al., 2013). This is the central reason mixed stars can exceed the pure-neutron-star limit when the dark fermion is lighter than the neutron.

The GR two-fluid formulation is standard in this literature. For a mixed star composed of baryons and dark fermions, the stress tensor is written as

$\Omega_{\rm DM}/\Omega_B \simeq 5$1

with each fluid separately conserved, and the hydrostatic equations become two coupled TOV equations driven by the same total enclosed mass and total pressure (Goldman et al., 2013). In one mirror-like example with $\Omega_{\rm DM}/\Omega_B \simeq 5$2, the authors find a mixed-star configuration with total mass $\Omega_{\rm DM}/\Omega_B \simeq 5$3, neutron-sphere radius $\Omega_{\rm DM}/\Omega_B \simeq 5$4 km, total radius $\Omega_{\rm DM}/\Omega_B \simeq 5$5 km, and a dark mass component $\Omega_{\rm DM}/\Omega_B \simeq 5$6 (Goldman et al., 2013). This is not a small-impurity scenario; it is a genuinely dark-matter-heavy compact object.

Self-interactions modify the stellar sequence in qualitatively different ways depending on whether they are repulsive or attractive. In the relativistic mean-field treatment of a Dirac spin-$\Omega_{\rm DM}/\Omega_B \simeq 5$7 ADM particle $\Omega_{\rm DM}/\Omega_B \simeq 5$8 with scalar attraction and vector repulsion,

$\Omega_{\rm DM}/\Omega_B \simeq 5$9

the energy density and pressure are

$Z$0

$Z$1

A major result is that strong scalar attraction increases, rather than decreases, the maximum stable ADM-star mass relative to free fermions; attractive self-interactions do not make fermionic ADM significantly better at destabilizing neutron stars than non-interacting fermionic ADM (Gresham et al., 2018).

The hybrid-star study of self-interacting asymmetric dark matter adopts a simpler benchmark with $Z$2, vector-mediator mass $Z$3, and a repulsive Yukawa interaction added to the degenerate Fermi-gas equation of state: $Z$4

$Z$5

Combined with a DDM3Y baryonic EoS and a two-fluid GR treatment, this yields pure self-interacting ADM stars with $Z$6, $Z$7 in the static case and $Z$8, $Z$9 in the rotating case (Mukhopadhyay et al., 2016).

The same study also finds that the headline mixed-star configuration is dark-matter dominated. For fixed nuclear central enthalpy and varying dark central enthalpy, the maximum occurs for differential rotation with DM frequency $\chi$0 Hz and nuclear frequency $\chi$1 Hz: $\chi$2 with mass decomposition

$\chi$3

The paper explicitly cautions that this is not an ordinary neutron star with a small ADM impurity; it is a dark-matter-dominated hybrid configuration (Mukhopadhyay et al., 2016). That caution matters because the mass–radius point alone does not reveal the composition.

5. Neutron-star capture, ADM cores, and collapse bounds

Neutron stars probe fermionic ADM in two distinct regimes. One is the equilibrium-structure regime, where a dark core or mixed fluid changes global stellar properties. The other is the capture-to-collapse regime, where nonannihilating ADM accumulates over gigayear timescales and may form a black hole.

In the equilibrium-inference regime, Bayesian analyses that vary both the baryonic and ADM equations of state simultaneously find that currently available mass–radius data constrain only the combination

$\chi$4

and only through a lower bound. Using NICER mass–radius data, the lower bound is

$\chi$5

$\chi$6

At the same time, $\chi$7, $\chi$8, and the ADM mass fraction $\chi$9 remain essentially unconstrained, and stars with $3/2$0 are nearly indistinguishable from purely baryonic stars for mass–radius uncertainties $3/2$1 (Rutherford et al., 2024). This indicates that small fermionic ADM cores are observationally subtle in mass–radius space.

The collapse literature asks a different question: whether captured fermionic ADM can exceed its Chandrasekhar limit and form a black hole inside an old neutron star. A conservative TOV-based study had already argued that captured spin-$3/2$2 ADM—whether self-attractive, repulsive, or noninteracting—cannot destroy neutron stars over cosmic times unless the ADM mass is roughly PeV scale or larger, because the ADM core must itself exceed the maximum stable mass of an ADM star with the same equation of state (Gresham et al., 2018). In that treatment, the maximal capture fraction from the DM flux through the star gives

$3/2$3

leading to $3/2$4 depending on the local dark-matter density (Gresham et al., 2018).

A more recent capture-to-collapse reassessment retains the same qualitative sequence—capture, thermalization, self-gravitation, Chandrasekhar collapse, black-hole formation, and competition between accretion and Hawking evaporation—but revises each step. The captured particle number is $3/2$5, the thermalized cloud is modeled by

$3/2$6

the thermal radius is

$3/2$7

and the fermionic collapse threshold becomes

$3/2$8

Because the analysis uses a Gaussian ADM distribution rather than a uniform sphere, and because it incorporates realistic NS capture, thermalization, accretion, and evaporation, previous results are relaxed by a few orders of magnitude (Robles et al., 30 Jul 2025). The revised bounds are relevant mainly for ultraheavy fermionic ADM, roughly

$3/2$9

rather than for canonical GeV-scale fermionic ADM (Robles et al., 30 Jul 2025).

A recurrent controversy concerns whether attractive self-interactions make fermionic ADM especially efficient at destabilizing neutron stars. The relativistic mean-field analysis states the opposite: attractive interactions soften the EoS only at lower densities, but by reducing the effective mass they accelerate the approach to the relativistic regime and do not substantially reduce the maximum stable ADM mass (Gresham et al., 2018). This controversy is therefore about the EoS treatment and the stability criterion, not merely about numerical details.

6. Detection channels, cosmological probes, and open disputes

The most constrained fermionic ADM scenarios are those in which the same operator both removes the symmetric component and mediates visible-sector scattering. In the heavy-mediator EFT with dimension-6 contact operators,

$B-L$00

the practical ADM requirement is

$B-L$01

so efficient annihilation imposes an upper bound on $B-L$02, while colliders and direct detection impose lower bounds (Roy et al., 2024). In that framework, quark-coupled Dirac-fermion ADM is strongly constrained throughout the preferred $B-L$03 range, while leptophilic ADM remains allowed for $B-L$04, and FCC-ee could improve the reach to $B-L$05 and $B-L$06 (Roy et al., 2024). The same study explicitly notes that excluding a region in $B-L$07 does not exclude all dark matter of mass $B-L$08, because light mediators can substantially change annihilation, collider kinematics, and direct-detection rates (Roy et al., 2024).

Indirect detection can probe the asymmetry-transfer operator itself. For fermionic ADM operators of the form

$B-L$09

the same interaction that shares asymmetry between sectors also induces late decays, with lifetime

$B-L$10

A distinctive ADM feature is that only one of $B-L$11 or $B-L$12 survives today, so the final-state charge asymmetry depends on the sign of the baryon or lepton number carried by the ADM particle (Zhao et al., 2014). The gamma-ray spectra are indifferent to that sign, but charged cosmic-ray signatures are not. Light ADM constraints correspond to $B-L$13, while heavy ADM fits to AMS-02 and H.E.S.S. point to $B-L$14, albeit still in tension with gamma rays and antiprotons (Zhao et al., 2014).

Cosmological hidden-sector probes remain important precisely because many viable fermionic ADM models suppress visible signatures. If the mediator decays into dark radiation, precise $B-L$15 measurements and DM–DR scattering become central observables rather than side effects (Blennow et al., 2012). In unified Dirac-neutrino/ADM models, the same FN-like structure that suppresses unwanted right-handed-neutrino interactions yields

$B-L$16

compatible with current bounds, while direct detection is loop-induced and future low-threshold experiments such as DS-LM-like searches can probe most of the remaining region (Ishida et al., 15 Oct 2025).

At the opposite end of the mass spectrum, asymmetric warm dark matter predicts a soft X-ray signal with helicity asymmetry. The radiative decay

$B-L$17

occurs at rate

$B-L$18

and because the dark matter is asymmetric and chiral, the emitted photons are circularly polarized (Yin et al., 2024). The paper describes helical X-rays as a smoking-gun signal of that scenario (Yin et al., 2024). This is a reminder that fermionic ADM phenomenology is not confined to the canonical GeV-scale direct-detection paradigm.

Several disputes in the field are now comparatively well defined. One is whether neutron-star survival generically excludes fermionic ADM; current results indicate that this is model-dependent and typically weak except for ultraheavy ADM (Gresham et al., 2018, Robles et al., 30 Jul 2025). Another is whether high-mass compact stars containing ADM should be interpreted as ordinary neutron stars with a small admixture; some explicit mixed-star solutions are instead dark-matter dominated and only mimic neutron-star masses and radii (Mukhopadhyay et al., 2016). A third is whether present collider and direct-detection exclusions rule out fermionic ADM as such; the EFT answer is severe for heavy-mediator couplings to Standard Model fermions, but hidden-sector annihilation, light mediators, composite realizations, and dark-radiation cosmologies remain open (Roy et al., 2024, Blennow et al., 2012).

Fermionic ADM is therefore best understood not as a single model but as a research program organized around a common relic principle and a common microscopic ingredient: a non-self-conjugate or effectively non-self-conjugate fermion carrying a primordial asymmetry. Across that program, the central technical themes are the removal of the symmetric component, the preservation or late violation of dark number, the role of Fermi degeneracy in dense matter, and the fact that many observables constrain only effective parameter combinations rather than the underlying microphysics.