A Single Sterile Neutrino Could Be Dark Matter and Help Build the Universe’s Matter

One of the deepest puzzles in modern physics is that three of the biggest open questions in the standard model of particle physics may share a single answer: right-handed neutrinos. These hypothetical partners of the familiar neutrinos carry no electric charge, feel no strong or electromagnetic force, and interact with ordinary matter only through gravity and whatever feeble channels a new theory provides. A new theoretical study published in The European Physical Journal C by Daijiro Suematsu of Kanazawa University argues that, within a well-studied extension of the standard model called the scotogenic model, the lightest of three right-handed neutrinos can serve as dark matter while its heavier siblings generate the cosmic baryon asymmetry, the tiny excess of matter over antimatter that allows galaxies, stars and people to exist at all.

The starting point is a set of facts the standard model simply cannot accommodate. Neutrino oscillation experiments have established beyond doubt that neutrinos have nonzero mass, something the standard model forbids. Observations of the abundances of light elements forged in the Big Bang, together with measurements of the cosmic microwave background, show that the early universe contained slightly more baryons than antibaryons, another phenomenon the standard model cannot produce. And a wealth of astrophysical and cosmological evidence points to dark matter, an invisible substance that outweighs ordinary matter roughly five to one, for which the standard model offers no candidate at all. Extending the model with right-handed neutrinos that carry Majorana masses has long been known to address the first two problems: the seesaw mechanism explains why observed neutrinos are so light, and leptogenesis, the out-of-equilibrium decay of heavy right-handed neutrinos, can seed the baryon asymmetry, provided those masses exceed roughly a billion giga-electronvolts.

What makes the new work striking is the claim that all three problems can be solved at the relatively low energy scale of a few tera-electronvolts, within the original scotogenic model and without adding any new interactions. The scotogenic model, introduced by Ernest Ma in 2006, extends the standard model with three right-handed neutrinos and an extra inert doublet scalar, both assigned odd parity under an imposed Z2 symmetry while all ordinary particles carry even parity. Because the inert doublet acquires no vacuum expectation value, neutrino masses vanish at tree level. Instead, they arise radiatively through a one-loop diagram in which the right-handed neutrinos couple to leptons and the inert scalar, with the small parameter lambda-5 in the scalar potential controlling the resulting tiny masses. The same Z2 symmetry renders the lightest odd particle absolutely stable, a built-in dark matter candidate.

Previous attempts to make the lightest right-handed neutrino the dark matter in this framework ran into two serious obstacles. First, if the dark matter particle annihilates efficiently enough to leave the correct relic abundance in the standard freezeout scenario, its neutrino Yukawa couplings must be of order one, and such large couplings induce lepton-flavor-violating processes such as the decay of a muon into an electron and a photon at levels already excluded by experiment. Second, generating sufficient lepton number asymmetry from right-handed neutrinos with masses of only a few tera-electronvolts is notoriously difficult, because the CP asymmetry in their decays is typically too small at that scale. Many authors have therefore extended the model with extra interactions, such as a new gauge symmetry, to circumvent these problems.

Suematsu’s solution exploits a detailed feature of the neutrino mass matrix that emerges when the diagonalization condition makes the mass eigenvalues expressible directly in terms of the Yukawa couplings. In the concrete example analyzed, the Yukawa texture yields the tribimaximal mixing pattern, with one neutrino mass eigenvalue exactly zero, and the charged-lepton sector supplies the small corrections needed to match oscillation data. Crucially, the CP asymmetry required for leptogenesis is proportional to the imaginary part of a combination of Yukawa couplings, which vanishes if those couplings are real. The analysis shows that complex Yukawa couplings, or the special case with a zero mass eigenvalue combined with Majorana phases in the right-handed neutrino masses, allow a nonzero asymmetry. A numerical benchmark with the two heavier right-handed neutrinos and the neutral component of the inert doublet all near three tera-electronvolts reproduces the measured mass-squared differences of neutrino oscillations while keeping lepton-flavor-violating decays safely below current bounds.

The dark matter abundance is handled through a mechanism quite different from the usual freezeout. Because the lightest right-handed neutrino’s Yukawa coupling is extremely small, it never reaches thermal equilibrium; instead it is produced by the gradual, so-called freeze-in process, mainly through the decay of the next-lightest inert scalar component, which itself is kept in equilibrium by standard model interactions and coannihilation. Solving the coupled Boltzmann equations for the number densities shows that the final dark matter abundance can be tuned to the observed value, corresponding to a density parameter of about 0.12, by choosing the small Yukawa coupling appropriately for dark matter masses ranging from a few giga-electronvolts up to the tera-electronvolt scale, and even down to the kilo-electronvolt range. When the dark matter mass approaches that of its decaying mother particle, the scalar quartic couplings lambda-3 and lambda-4, which control the coannihilation rate, become essential dials, and the analysis finds that a dark matter mass of at least about 8.5 giga-electronvolts is needed in the simplest case.

The leptogenesis side relies on an elegant trick: a strict near-degeneracy between the second-heaviest right-handed neutrino and the neutral inert scalar, with their masses differing by a fraction of order one hundred-thousandth or less. This degeneracy suppresses the decay width of the heavier neutrino through phase-space factors, delaying its decay until it falls far out of thermal equilibrium, exactly the condition leptogenesis requires. Meanwhile, a tiny mass splitting with the heaviest right-handed neutrino, of order one part in a billion, resonantly enhances the CP asymmetry through the self-energy diagram, boosting it to the level of ten to the minus seven needed to generate the observed lepton asymmetry of order ten to the minus ten. Quantum corrections to the right-handed neutrino masses, calculated at two loops, turn out to be far too small to spoil this delicate degeneracy, so no extra symmetry is needed to protect it.

The full numerical treatment, tracking the number densities of the decaying neutrino, the lepton asymmetry, the inert scalar and the dark matter candidate simultaneously, confirms that the scenario holds together. The late-time decay of the second-heaviest neutrino injects additional inert scalars into the plasma, which eventually decay into dark matter and modify the final abundance, so the couplings must be retuned compared with the simplified analysis. The study shows this retuning works across the mass range: for dark matter at 150 giga-electronvolts or 1.5 giga-electronvolts, a suitably small Yukawa coupling delivers the right abundance, while for dark matter near the tera-electronvolt scale the quartic couplings must be adjusted so that the combined abundance of the scalar and the dark matter particle converges to the required value. Notably, because the dark matter is the right-handed neutrino itself, direct detection experiments impose no constraint on the scalar quartics, a sharp contrast with the case where the inert scalar is the dark matter.

The scenario also carries testable consequences. The charged and neutral components of the inert doublet, with masses near three tera-electronvolts, could be produced at future colliders, where processes yielding multiple leptons plus missing transverse energy, displaced vertices from long-lived charged scalars, or deviations in the Higgs decays to two photons and to a photon plus a Z boson could distinguish this model from the ordinary inert doublet model. Predictions for the effective masses governing neutrinoless double beta decay and beta decay come out far below present experimental sensitivity, and the induced lepton-flavor-violating branching ratios and the electron electric dipole moment are similarly tiny, making them difficult but not impossible targets for next-generation searches. If the picture is right, the three great mysteries left unsolved by the standard model would trace back to a single family of particles, with the universe’s missing mass and its matter-antimatter imbalance written into the same small set of couplings, and the decisive evidence waiting in the debris of high-energy collisions rather than in a deep underground detector.

Subject of Research: Right-handed neutrino dark matter and leptogenesis in the scotogenic model

Article Title: Right-handed neutrino dark matter consistent with the generation of baryon number asymmetry

Article References: Suematsu, D. (2026). Right-handed neutrino dark matter consistent with the generation of baryon number asymmetry. The European Physical Journal C, 86(10), Article 1143. https://doi.org/10.1140/epjc/s10052-026-16425-1

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16425-1

Keywords: dark matter, right-handed neutrinos, scotogenic model, leptogenesis, baryon asymmetry, neutrino masses, seesaw mechanism, freeze-in, inert doublet, Z2 symmetry, CP asymmetry, particle physics