A shared origin for dark matter and the universe’s matter imbalance?

Dark matter’s present-day energy density is about 5.4 times that of ordinary baryonic matter. That relationship motivates a question: could their abundances have a common origin? In their theoretical study, Hiroki Takahashi and Juntaro Wada propose a mechanism that produces an excess of ordinary matter over antimatter alongside an excess of dark matter particles over their antiparticles.

The proposal belongs to a class of models called asymmetric dark matter. Here, the surviving dark matter abundance is determined mainly by a particle–antiparticle imbalance, rather than by equal populations of both. The researchers calculate how two such imbalances could arise together. They do not report a detection of new particles.

A changing field supplies the bias

The model extends a framework used to explain neutrino masses. It includes hypothetical right-handed neutrinos, a dark matter fermion, a heavier dark-sector scalar particle and a dark photon. Another scalar field breaks a symmetry associated with the difference between baryon and lepton number. Its angular component is called the majoron.

The majoron’s motion is central. The authors assume an early-universe state with substantial motion in this angular direction, known as kinetic misalignment. The resulting time-dependent background shifts particle and antiparticle energies differently. When suitable number-changing reactions operate, that difference can generate unequal populations.

Ordinary leptons and Higgs particles combine to form right-handed neutrinos through reactions called inverse decays. Dark matter particles and the additional dark scalar undergo corresponding reactions. Both respond to the same majoron background. Electroweak processes called sphalerons subsequently convert part of the ordinary-sector lepton imbalance into a baryon imbalance.

Unlike conventional thermal leptogenesis, this mechanism can generate the imbalance while the right-handed neutrinos remain in thermal equilibrium. The authors argue that it permits neutrino masses far below the roughly billion-GeV scale associated with conventional hierarchical thermal leptogenesis, without requiring nearly identical neutrino masses to enhance the effect.

Two ways to preserve an imbalance

The researchers numerically solve Boltzmann equations, which track how reactions and cosmic expansion change particle abundances. Their benchmark calculations use lightest right-handed neutrino masses of 3,000, 30,000 and 300,000 GeV. They adjust the majoron background to reproduce the observed baryon asymmetry, then calculate the dark matter mass needed to match its observed abundance.

The key variable is the interaction strength connecting dark matter to the lightest right-handed neutrino.

For stronger interactions, the dark matter asymmetry reaches its equilibrium value before the relevant inverse decays become ineffective. The authors call this the freeze-out regime. Both ordinary and dark equilibrium asymmetries depend on the majoron’s angular speed and the temperature squared. Their shared source therefore connects the two abundances, although their different decoupling times also matter.

Stronger coupling does not simply produce a larger surviving imbalance. In this regime, it keeps reactions active longer, until expansion has reduced the majoron’s angular speed. The final asymmetry becomes smaller, requiring heavier dark matter to reproduce the same energy density.

For weaker interactions, the dark asymmetry never reaches equilibrium: the freeze-in regime. Here, weaker coupling produces less asymmetry, again requiring heavier dark matter. Importantly, this describes freeze-in of the asymmetry; the analysis still assumes that dark-sector particles share the ordinary sector’s temperature.

The authors summarize the typical mass range as roughly 0.1–100 GeV for freeze-out, extending to roughly 10 TeV for freeze-in. Their numerical analysis finds a minimum near 0.5 GeV around the transition between regimes. These are model-dependent predictions, not measured masses.

Conditions and possible tests

Producing an imbalance is insufficient by itself. The remaining particle–antiparticle pairs must annihilate efficiently so that they do not dominate the dark matter abundance. The model uses annihilation into lighter dark photons for this purpose. Requiring the heavier dark scalar’s abundance to remain negligible, while keeping the interaction strength within the calculation’s perturbative range, helps set the approximately 10-TeV upper mass scale.

Other assumptions restrict the results. The calculations focus on couplings small enough that additional asymmetry-changing scattering reactions can be neglected. They assume that the dark scalar’s own asymmetry is rapidly erased and that the majoron does not significantly alter the universe’s expansion history. Some calculations also neglect dark-particle masses in reaction rates, limiting their applicability near production thresholds.

Dark photons can connect the model to direct-detection searches through mixing with ordinary photons. But a candidate’s mass alone does not guarantee detectability: scattering also depends on the dark photon’s mass and interaction strengths. The study identifies a potentially testable shared-origin mechanism, not evidence that this mechanism actually operated in the universe.