Dark-matter sterile neutrinos in models with a gauge singlet in the Higgs sector

TL;DR

The paper explores a beyond-Standard-Model framework where a gauge-singlet scalar S induces Majorana masses for right-handed neutrinos, enabling keV-scale sterile neutrino dark matter produced via S decays at high temperatures. It analyzes both equilibrium and out-of-equilibrium Higgs-decay production channels, showing that entropy dilution and the Higgs-sector dynamics can yield colder dark matter with relaxed Lyman-α bounds. Finite-temperature two-field potential analyses reveal scenarios with strong first-order electroweak phase transitions, offering avenues for electroweak baryogenesis and promises of collider signatures at the LHC and ILC. The work also discusses how low-scale leptogenesis via neutrino oscillations could generate the baryon asymmetry, linking dark matter production, phase transition history, and baryogenesis within a single Higgs-portal framework.

Abstract

Sterile neutrino with mass of several keV can be the cosmological dark matter, can explain the observed velocities of pulsars, and can play an important role in the formation of the first stars. We describe the production of sterile neutrinos in a model with an extended Higgs sector, in which the Majorana mass term is generated by the vacuum expectation value of a gauge-singlet Higgs boson. In this model the relic abundance of sterile neutrinos does not necessarily depend on their mixing angles, the free-streaming length can be much smaller than in the case of warm dark matter produced by neutrino oscillations, and, therefore, some of the previously quoted bounds do not apply. The presence of the gauge singlet in the Higgs sector has important implications for the electroweak phase transition, baryogenesis, and the upcoming experiments at the Large Hadron Collider and a Linear Collider.

Figures (7)

• Figure 1: The variation of the $S$ boson freeze-out parameter $r_{\rm f}=m_S/T_{\rm f}$ with the coupling to SM particles $\lambda_{HS}$. For numerical estimations we used $m_S=200\: {\rm GeV}$.
• Figure 2: Contour plots of $V_{\mathrm{eff}}(\eta,\sigma,T)$ at $T\gg T_c$, $T=T_c$, and $T=0$, corresponding to the parameter set A of table \ref{['data']}. At $T\gg T_c$ the universe is in the unbroken phase $\eta=0$, with $\sigma>0$. At lower temperatures, the minimum of $V_{\rm eff}$ shifts to non-zero $\eta$, while a second minimum appears in the $\sigma<0$ region. The two minima become degenerate at $T=T_c$, and at $T=0$ the true vacuum is located at $\eta=246 \: {\rm GeV}$, $\sigma <0$.
• Figure 3: The potential configuration along the straight-line path connecting the two minima, at various temperatures. At very high T the potential possesses only one minimum. At a lower temperature $T_1$ a local minimum starts forming. At $T_c<T_1$ the two minima become degenerate. At $T_o<T_c$ tunneling to the true vacuum occurs. At $T=0$ the universe has settled in the true vacuum. The curves correspond to parameter set A of table \ref{['data']}.
• Figure 4: The evolution of the SM Higgs VEV $\eta$ with temperature. At very high T, symmetry is restored: $\eta=0$. At a lower temperature a second order phase transition to $\eta \neq 0$ takes place, while still remaining in the false vacuum (dashed line). At $T=T_o$, a first order phase transition brings the universe to the true vacuum (solid line). At $T=0$, $\eta=246 \: {\rm GeV}$. The data corresponds to parameter set A of table \ref{['data']}.
• Figure 5: The free energy density vs temperature, at the true vacuum (lower curve) and at the false vacuum (upper curve). Significant reheating could occur if the universe cooled down to a low temperature before the tunneling took place. At the temperature at which the tunneling actually occurs, $T\approx 100 \: {\rm GeV}$, the reheating is not significant.