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Register a new userScalar assisted singlet doublet fermion dark matter model and electroweak vacuum stability
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We extend the so-called singlet doublet dark matter model, where the dark matter is an admixture of a Standard Model singlet and a pair of electroweak doublet fermions, by a singlet scalar field. The new portal coupling of it with the dark sector not only contributes to the dark matter phenomenology (involving relic density and direct detection limits), but also becomes important for generation of dark matter mass through its vacuum expectation value. While the presence of dark sector fermions affects the stability of the electroweak vacuum adversely, we find this additional singlet is capable of making the electroweak vacuum absolutely stable upto the Planck scale. A combined study of dark matter phenomenology and Higgs vacuum stability issue reflects that the scalar sector mixing angle can be significantly constrained in this scenario.
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We consider singlet extensions of the standard model, both in the fermion and the scalar sector, to account for the generation of neutrino mass at the TeV scale and the existence of dark matter respectively. For the neutrino sector we consider models with extra singlet fermions which can generate neutrino mass via the so called inverse or linear seesaw mechanism whereas a singlet scalar is introduced as the candidate for dark matter. We show that although these two sectors are disconnected at low energy, the coupling constants of both the sectors get correlated at high energy scale by the constraints coming from the perturbativity and stability/metastability of the electroweak vacuum. The singlet fermions try to destabilize the electroweak vacuum while the singlet scalar aids the stability. As an upshot, the electroweak vacuum may attain absolute stability even upto the Planck scale for suitable values of the parameters. We delineate the parameter space for the singlet fermion and the scalar couplings for which the electroweak vacuum remains stable/metastable and at the same time giving the correct relic density and neutrino masses and mixing angles as observed.
By extending the Standard Model with singlet-doublet fermions and triplet scalars, all odd under a new $Z_2$ symmetry, we introduce a radiative seesaw model that can simultaneously account for dark matter, explain the existence of neutrino masses and allow for gauge coupling unification. We explore the viable parameter space of the model after imposing collider, Higgs mass, dark matter, neutrino mass and lepton flavour violation constraints. We find that dark matter in this model is fermionic for masses below about 1 TeV and scalar above and observe a high degree of complementarity between direct detection and lepton flavour violation experiments, which should soon allow to fully probe the fermionic dark matter sector and at least partially the scalar dark matter sector.
In the framework of type-II two-Higgs-doublet model with a singlet scalar dark matter $S$, we study the dark matter observables, the electroweak phase transition, and the gravitational wave signals by such strongly first order phase transition after imposing the constraints of the LHC Higgs data. We take the heavy CP-even Higgs $H$ as the only portal between the dark matter and SM sectors, and find the LHC Higgs data and dark matter observables require $m_S$ and $m_H$ to be larger than 130 GeV and 360 GeV for $m_A=600$ GeV in the case of the 125 GeV Higgs with the SM-like coupling. Next, we carve out some parameter space where a strongly first order electroweak phase transition can be achieved, and find benchmark points for which the amplitudes of gravitational wave spectra reach the sensitivities of the future gravitational wave detectors.
One of the simplest viable models for dark matter is an additional neutral scalar, stabilised by a $mathbb{Z}_2$ symmetry. Using the GAMBIT package and combining results from four independent samplers, we present Bayesian and frequentist global fits of this model. We vary the singlet mass and coupling along with 13 nuisance parameters, including nuclear uncertainties relevant for direct detection, the local dark matter density, and selected quark masses and couplings. We include the dark matter relic density measured by Planck, direct searches with LUX, PandaX, SuperCDMS and XENON100, limits on invisible Higgs decays from the Large Hadron Collider, searches for high-energy neutrinos from dark matter annihilation in the Sun with IceCube, and searches for gamma rays from annihilation in dwarf galaxies with the Fermi-LAT. Viable solutions remain at couplings of order unity, for singlet masses between the Higgs mass and about 300 GeV, and at masses above $sim$1 TeV. Only in the latter case can the scalar singlet constitute all of dark matter. Frequentist analysis shows that the low-mass resonance region, where the singlet is about half the mass of the Higgs, can also account for all of dark matter, and remains viable. However, Bayesian considerations show this region to be rather fine-tuned.
We revisit the theory and phenomenology of scalar electroweak multiplet thermal dark matter. We derive the most general, renormalizable scalar potential, assuming the presence of the Standard Model Higgs doublet, $H$, and an electroweak multiplet $Phi$ of arbitrary SU(2$)_L$ rank and hypercharge, $Y$. We show that, in general, the $Phi$-$H$ Higgs portal interactions depend on three, rather than two independent couplings as has been previously considered in the literature. For the phenomenologically viable case of $Y=0$ multiplets, we focus on the septuplet and quintuplet cases, and consider the interplay of relic density and spin-independent direct detection cross section. We show that both the relic density and direct detection cross sections depend on a single linear combination of Higgs portal couplings, $lambda_{rm eff}$. For $lambda_{rm eff}sim mathcal{O}(1)$, present direct detection exclusion limits imply that the neutral component of a scalar electroweak multiplet would comprise a subdominant fraction of the observed DM relic density.
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