Asymmetric Dark Matter from Leptogenesis

TL;DR

The paper proposes a two-sector leptogenesis framework in which right-handed neutrinos couple to both the SM and a hidden dark sector, generating correlated lepton and dark matter asymmetries. By solving Boltzmann equations and mapping washout and transfer effects, the authors show that the dark matter mass can span from keV to 10 TeV, with distinctive phenomenology including late-time regeneration of symmetric DM, sterile-neutrino-like DM from hidden-sector mixing, and potential indirect detection signals. It also discusses how a full SM-plus-hidden-sector realization modifies the Davidson–Ibarra bound and yields rich cosmological scenarios for light DM, including mixed warm/cold components. The work highlights the broad landscape of ADM predictions under two-sector leptogenesis and outlines several viable variants with observational consequences across cosmology, astrophysics, and particle physics.

Abstract

We present a new realization of asymmetric dark matter in which the dark matter and lepton asymmetries are generated simultaneously through two-sector leptogenesis. The right-handed neutrinos couple both to the Standard Model and to a hidden sector where the dark matter resides. This framework explains the lepton asymmetry, dark matter abundance and neutrino masses all at once. In contrast to previous realizations of asymmetric dark matter, the model allows for a wide range of dark matter masses, from keV to 10 TeV. In particular, very light dark matter can be accommodated without violating experimental constraints. We discuss several variants of our model that highlight interesting phenomenological possibilities. In one, late decays repopulate the symmetric dark matter component, providing a new mechanism for generating a large annihilation rate at the present epoch and allowing for mixed warm/cold dark matter. In a second scenario, dark matter mixes with the active neutrinos, thus presenting a distinct method to populate sterile neutrino dark matter through leptogenesis. At late times, oscillations and dark matter decays lead to interesting indirect detection signals.

Figures (9)

• Figure 1: A schematic view of our framework: the SM and DM sectors are indirectly connected via Yukawa interactions with the same heavy right-handed neutrinos, $N_i$. The complex couplings, $\lambda_i$ and $y_i$, lead to CP violation in $N_i$ decays, and consequently particle-antiparticle asymmetries for DM and leptons.
• Figure 2: Feynman diagrams contributing to the 2-to-2 terms in the Boltzmann Eqs. \ref{['eq:BE2']} and \ref{['eq:BE3']}, that transfer the lepton asymmetry between the two sectors. The top row shows diagrams that violate lepton number, while the transfer diagrams in the bottom row conserve lepton number.
• Figure 3: Solutions to the Boltzmann equations for a 2-sector toy model with both sectors in the weak washout regime, $\Gamma_{N_1}{\rm Br}_{l,\chi}/H_1\ll1$ (and consequently in the narrow-width limit), assuming the initial condition $Y_{N_1} = Y_{N_1}^{eq}$. In this limit, the washout efficiencies are $\eta_{l,\chi}=1$ and the final lepton and DM abundances depend only on $\epsilon_{l,\chi}$ as in Eq. \ref{['eq:eta']}. The left plot shows the $N_1$ abundance (purple line) as a function of $z=M_{N_1}/T$, with its equilibrium value, $Y_{N_1}^{\rm eq}$, plotted for reference (black dashed). The right plot shows the asymmetry abundances normalized to the asymptotic lepton abundance for ${\Delta L}$ (blue dashed) and ${\Delta\chi}$ with $m_\chi =$ keV (red dotted) and $m_\chi=10$ TeV (red line).
• Figure 4: Solutions to the Boltzmann equations in the case where both sectors are in the weak washout regime $\Gamma_{N_1}{\rm Br}_{L,\chi}/H_1\ll1$ (which implies the narrow-width approximation), for ${\rm Br}_\chi = 10^{-2}$ and $\epsilon_{L,\chi} = 10^{-5}\times {\rm Br}_{L,\chi}$. Two solutions are shown, assuming thermal and zero initial conditions for $N_1$. The former implies washout efficiency of order one, $\eta_{L,\chi}=1$ as in Fig. \ref{['fig:weakweak']}. On the other hand, in the latter case $N_1$ never thermalizes and consequently the efficiencies are smaller, as predicted in Eq. \ref{['eq:weakweakeff']}. The left plot shows the ratio of lepton to dark matter abundance as a function of $z=M_{N_1}/T$ while the right plot shows the normalized $N_1$ abundance.
• Figure 5: Normalized abundances of lepton and DM asymmetries as a function of $z=M_{N_1}/T$. The dashed curves show the expected asymptotic asymmetries for unit washout efficiencies, $\eta_{L,\chi}=1$. The left plot shows the solution in the case $\Gamma_{N_1}/H_1 =1$. The DM asymmetry changes sign due to the significant washout and transfer effects. The final asymmetry in that sector is greater than one. Here $\Gamma_{N_1}/M_{N_1} = 0.1$ and ${\rm Br}_L=0.9$. On the right plot, the solution is shown with identical parameters except for $\Gamma_{N_1}/H_1=50$. The corresponding theory is in the strong/strong regime with wide $N_1$ width. As can be seen, the large washout and transfer effects reverse the ratio of lepton to DM abundance, rendering a larger number density in the dark sector. As discussed in Sec. \ref{['sec:washout']} the ratio of the asymptotic abundances is independent of $\epsilon_{L,\chi}$.