Aidnogenesis via Leptogenesis and Dark Sphalerons

We discuss aidnogenesis, the generation of a dark matter asymmetry via new sphaleron processes associated to an extra non-abelian gauge symmetry common to both the visible and the dark sectors. Such a theory can naturally produce an abundance of asymmetric dark matter which is of the same size as the lepton and baryon asymmetries, as suggested by the similar sizes of the observed baryonic and dark matter energy content, and provide a definite prediction for the mass of the dark matter particle. We discuss in detail a minimal realization in which the Standard Model is only extended by dark matter fermions which form “dark baryons” through an SU(3) interaction, and a (broken) horizontal symmetry that induces the new sphalerons. The dark matter mass is predicted to be approximately 6 GeV, close to the region favored by DAMA and CoGeNT. Furthermore, a remnant of the horizontal symmetry should be broken at a lower scale and can also explain the Tevatron dimuon anomaly.

💡 Research Summary

The paper introduces the concept of “aidnogenesis,” a mechanism whereby the dark‑matter asymmetry is generated through new sphaleron processes associated with an extra non‑abelian gauge symmetry that couples both the visible and dark sectors. The authors start by noting that conventional asymmetric dark‑matter (ADM) models typically rely on a direct transfer of a lepton asymmetry into a dark‑matter asymmetry. In contrast, they propose that a horizontal (flavor‑type) gauge group, denoted SU(2)ₕ (or more generally SU(N)ₕ), is broken at a high temperature in the early Universe. The breaking activates “dark sphalerons,” which violate the combined quantum numbers of baryon number (B), lepton number (L), and a new dark‑baryon number (D) simultaneously.

Using equilibrium thermodynamics and chemical‑potential relations, the authors derive the constraints imposed by the dark sphaleron processes. The key result is a fixed ratio among the asymmetries: B : L : D = (28/79) : 1 : −(28/79). This relation implies that, once a lepton asymmetry is generated (for example by standard leptogenesis), the sphalerons automatically distribute it between ordinary baryons and dark‑baryons in a predictable way. Because the observed cosmological densities satisfy Ω_DM ≈ 5 Ω_B, the ratio forces the dark‑matter particle mass to be of order a few GeV. In the minimal realization presented, the dark sector consists of three Weyl fermions χ_i (i = 1,2,3) charged under an SU(3)_D “dark colour’’ interaction. These fermions bind into stable “dark baryons” (χχχ) that serve as the dark‑matter candidates.

The model’s field content is deliberately minimal: the Standard Model (SM) is left unchanged except for the addition of the χ_i fields. The horizontal gauge group SU(2)ₕ couples the χ_i to SM leptons, providing the portal through which the dark sphalerons act. SU(2)ₕ is broken in two stages. The first stage occurs at a high scale Λₕ ≈ 10 TeV, where the sphaleron transitions are rapid (Γ_sph ≫ Hubble). The second stage occurs at a lower scale Λ′ₕ ≈ 1 TeV, leaving a massive Z′ gauge boson. This residual Z′ can generate flavour‑changing neutral currents that address the long‑standing Tevatron dimuon anomaly (an excess of CP‑violating dimuon events) by contributing to b → s μ⁺μ⁻ transitions.

The authors discuss several phenomenological constraints. Direct‑detection experiments (DAMA/LIBRA, CoGeNT) favour dark‑matter masses in the 5–10 GeV range, precisely where the model predicts the dark baryon mass (≈ 6 GeV). The scattering cross‑section is mediated either by Higgs‑portal scalar exchange or by kinetic mixing between the photon and a dark photon associated with the SU(3)_D sector; both mechanisms can be tuned to satisfy current limits while remaining within reach of upcoming experiments. Cosmological bounds from Big‑Bang Nucleosynthesis and the Cosmic Microwave Background are respected by keeping the kinetic‑mixing parameter sufficiently small, preventing excessive energy transfer between the sectors.

A crucial theoretical consistency check is the requirement that the dark sphaleron rate be fast enough before the electroweak sphalerons freeze out, yet cease well before today so that the generated asymmetries are preserved. By choosing the horizontal gauge coupling gₕ and the breaking scale Λₕ appropriately, the authors demonstrate that Γ_sph ∼ αₕ⁵ T ≫ H(T) for T ≈ Λₕ, while after symmetry breaking the sphaleron processes become exponentially suppressed.

In summary, the paper delivers three main achievements: (1) it provides a unified origin for the lepton, baryon, and dark‑matter asymmetries via a single set of sphaleron processes; (2) it predicts a dark‑matter mass around 6 GeV, naturally explaining the observed similarity between Ω_DM and Ω_B; and (3) it links the same horizontal gauge symmetry to low‑energy flavour anomalies, offering a potential explanation for the Tevatron dimuon excess. The framework is economical, experimentally testable, and opens a new avenue for connecting baryogenesis, dark‑matter physics, and flavour phenomenology.