Asymmetric Dark Matter via Spontaneous Co-Genesis

TL;DR

This work introduces a spontaneous co-genesis mechanism for asymmetric dark matter by coupling a light, ultra-low-mass scalar $\phi$ derivatively to DM, generating a DM asymmetry in tandem with baryon asymmetry through a rolling background and $X$-number violation. A sharing operator mediates the transfer of asymmetry to the visible sector, producing correlated baryon and DM abundances, while the same $\phi$ field also enables efficient annihilation of the symmetric DM component into light states. The analysis derives the required parameter relations, identifies viable regimes in the $T_X$–$T_S$ plane, and shows how cosmological and astrophysical constraints bound $m_\phi$, $T_X$, and related couplings. Overall, the mechanism links DM and baryon densities without new CP-violating sources and yields testable predictions for a sub-eV scalar mediator and ADM mass scales around the GeV–10 GeV range.

Abstract

We investigate, in the context of asymmetric dark matter (DM), a new mechanism of spontaneous co-genesis of linked DM and baryon asymmetries, explaining the observed relation between the baryon and DM densities, Omega_DM/Omega_B ~ 5. The co-genesis mechanism requires a light scalar field, phi, with mass below 5 eV which couples derivatively to DM, much like a `dark axion'. The field phi can itself provide a final state into which the residual symmetric DM component can annihilate away.

Figures (3)

• Figure 1: Left panel: The relic density of ADM energy density as a function of the DM mass for $\alpha^2 \phi_0/f = 1$ and $T_X = 10^{10}$ GeV. The black line corresponds to the full solution calculated using eq.(\ref{['eq:k']}) and the red dashed (blue dotted) line corresponds to the relativistic (non-relativistic) approximation given in in eq.(\ref{['eq:k2']}). Right panel: The required values of the scalar field parameter combination $\alpha^2 \phi_0/f$ for varying DM mass at contours of fixed $T_X$, where the value of $T_X$ in GeV is labelled on each line. As damped motion requires $\alpha < 1$, low values of $T_X\lesssim 10^5$ GeV require large values of $\phi_0/f$.
• Figure 2: Solutions, satisfying $\Omega_X/\Omega_B = 4.97$, for the ADM mass, $M_X$, as a function of the sharing freeze-out temperature, $T_S$. We assume the sharing operator of eq.(\ref{['eq:sharing']}), and complete annihilation of the symmetric component of the DM density. We illustrate a typical electroweak phase transition temperature by the vertical green dashed line, and a representative temperature, $T_{sph}$, at which sphalerons have become inactive, by the vertical blue dashed line. For a given $T_S \gtrsim 20$ GeV there are two successful ADM solutions: One for $M_X \simeq 10$ GeV, where the DM is relativistic at $T_S$, while the other, non-relativistic solution has $M_X$ increasing with $T_S$ (as the DM density is Boltzmann suppressed in the non-relativistic regime). This is the 'sharing' paradigm.
• Figure 3: Contours of constant $M_X$ in the $T_X$--$T_S$ plane corresponding to the generation of $\Omega_X/\Omega_B = 4.97$, and $\Omega_B h^2 =0.023$. For $T_S < T_X$ there are two branches of solutions corresponding to the two branches shown in Figure \ref{['fig:TXgtrTSmass']}: The first, relativistic solution occurs when $M_X \sim 10$ GeV and fills the entire lower half plane (shaded region), while the second branch is shown by the horizontal contours in the lower half plane labelled by $M_X$ in units of GeV. Both solutions are independent of $T_X$ as in this case the sharing of the asymmetry is determined after the total asymmetry has been frozen in. On the other hand, for $T_X < T_S$ the DM asymmetry continues to evolve after sharing has ceased. The resulting contours of constant $M_X$ corresponding to successful generation of $\Omega_X/\Omega_B$, and $\Omega_B h^2$ are shown in the upper half plane, and the mass, $M_X$, now depends on both $T_X$ and $T_S$. The portions of contours where $T_S \propto T_X$ apply for the relativistic case ($M_X\ll T_X$), while the remaining portions apply to the semi- and non-relativistic cases ($M_X\gtrsim T_X$). The solution for $M_X \sim 10$ GeV lies along the line $T_S \simeq T_X$, and hence every point of this line corresponds to a solution when we continue to the $T_S < T_X$ corner, showing the continuity between solutions on either side of the line $T_X = T_S$. By allowing the DM asymmetry to evolve after sharing has ceased, a new set of solutions for a given DM mass and $T_S$ open up in the upper left half plane, in addition to the standard solutions in the lower half plane where $T_S<T_X$.