Dominant Thermal Resonant Mechanism for Low-Scale Leptogenesis

Abstract

We explicitly demonstrate the importance of a new thermal resonant channel in the context of low-scale leptogenesis, which goes beyond the well-known mixing and oscillation of massive singlet neutrinos. This new channel is always present when considering the thermally-induced Higgs decay to leptons and relativistic sterile neutrinos, and can become dominant thanks to thermally-generated lepton-doublet flavour oscillations. This mechanism can yield the observed baryon asymmetry in our universe, even if there is no resonant enhancement from quasi-degenerate sterile neutrinos. The required active-to-sterile neutrino mixing differs from the other two low-scale leptogenesis channels and can be probed in fixed-target and long-lived particle experiments, and by displaced vertex searches at high-energy colliders.

Figures (2)

• Figure 1: Tree-level and one-loop Higgs decay to leptons and relativistic sterile neutrinos, where the red, thick lines represent the thermal corrections. The free Higgs undergoes coherent forward scattering (CFS) with the background plasma, acquiring a thermal mass to trigger the decay before gauge symmetry breaking. The produced lepton doublets also undergo CFS, enhancing the absorptive part of the loop amplitude induced by the thermal cut (blue line).
• Figure 2: The parameter space of low-scale leptogenesis from the third channel presented in this Letter. We fix the lightest SM neutrino mass at 0.01 eV and consider muon dominance $\theta_\mu^2\gg \theta_e^2$. The green shaded regions correspond to $Y_B=[10^{-11},10^{-10}]$, while the gray shaded regions are excluded by CMS and BBN. The dashed lines represent the projected detection limits.