Baryogenesis and Dark Matter from light Sterile Neutrinos

Abstract

We propose a simple and flexible mechanism by which sterile neutrinos with masses below the electroweak scale can simultaneously account for the observed baryon asymmetry of the Universe and the dark matter abundance. Crucially, neutrinos in this mass range behave as Dirac particles at high temperatures, allowing connections to Dirac leptogenesis, while at low temperatures, they can serve as viable warm dark matter candidates. We first perform a general analysis, assuming that unspecified ultraviolet dynamics generate both symmetric and asymmetric sterile-neutrino abundances before decoupling. Treating these abundances as initial conditions for the subsequent evolution allows us to systematically explore the phenomenologically viable regions of the low-energy parameter space, taking into account cosmological and astrophysical constraints, as well as implications for light-neutrino mass generation. Finally, we illustrate the model-building opportunities enabled by this minimal setup by studying two specific ultraviolet completions.

Figures (12)

• Figure 1: Illustration of the setup considered in this work. An unspecified UV completion of the type-I seesaw generates symmetric and asymmetric components of the sterile neutrino abundance at a temperature $T\gtrsim T_{\mathrm{p}}$ through CP-violating, out-of-equilibrium decays or oscillations. Part of the symmetric abundance, illustrated here by $Y_{N_R^h}$, may decay before today, while the surviving steriles may compose viable dark matter candidates. The asymmetric component, $Y_{\Delta N_R}$, sequesters lepton number and, with the aid of the weak sphaleron processes, allows for a partial conversion of the leptonic asymmetry $Y_{\Delta\nu_L}$ into a baryon asymmetry $Y_B$. This asymmetry remains until today, provided that the sterile neutrinos remain out of equilibrium until after the EWPT.
• Figure 2: Diagrammatic representation of s-channel processes contributing to left-right equilibration of neutrinos. Dashed lines represent scalars, wiggly lines gauge bosons, and solid lines fermions.
• Figure 3: Dominant decay diagram for keV sterile neutrinos. The cross resembles active-sterile mixing, and the final state corresponds to light SM neutrinos, with an electroweak Z-boson as mediator.
• Figure 4: Diagrams for sterile neutrino decay leading to monochromatic photon emission.
• Figure 5: Estimate of the contribution of the lightest sterile neutrino to $\Delta N_{\mathrm{eff}}$ at the temperature at which it becomes non-relativistic, assuming that it gives the full dark matter abundance. The gray and light gray shaded regions correspond to the current bound from the Planck collaboration (at $2\,\sigma$ C.L.) Planck:2018vyg and from Planck+ACT combined data ACT:2020gnv, respectively, and the dashed lines correspond to forecasts CMB-S4:2016pleSPT-3G:2014dbxMacInnis:2023vif.