Abstract
The simplest extension of the Standard Model by only one real singlet scalar can explain the observed dark matter relic density while giving simultaneously a strongly first-order electroweak phase transition in the early universe. However, after imposing the invisible Higgs decay constraint from the LHC, the parameter space of the single scalar model shrinks to regions with only a few percentage of the DM relic abundance and when adding the direct detection bound, e.g. from XENON100, it gets excluded completely. In this paper, we extend the Standard Model with two real gauge singlet scalars, here s and s′, and show that the electroweak symmetry breaking may occur via different channels. Despite very restrictive first-order phase transition conditions for the two-scalar model in comparison to the single scalar model, there is a viable space of parameters in different phase transition channels that simultaneously explains a fraction or the whole dark matter relic density, a strongly first-order electroweak phase transition and still evading the direct detection bounds from the latest LUX/XENON experiments while respecting the invisible Higgs decay width constraint from the LHC.
Highlights
The simplest extension of the Standard Model by only one real singlet scalar can explain the observed dark matter relic density while giving simultaneously a strongly first-order electroweak phase transition in the early universe
Despite very restrictive first-order phase transition conditions for the two-scalar model in comparison to the single scalar model, there is a viable space of parameters in different phase transition channels that simultaneously explains a fraction or the whole dark matter relic density, a strongly first-order electroweak phase transition and still evading the direct detection bounds from the latest LUX/XENON experiments while respecting the invisible Higgs decay width constraint from the LHC
The first and the simplest of such models is the extension of the SM with only one real single scalar which has been studied vastly in the literature, see e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] in which various phenomenological aspects such as the dark matter relic density extracted from WMAP and Planck [24,25,26,27], the Higgs invisible decay width from the LHC experiments [28, 29], the upper bound on the dark matter elastic scattering cross section off nuclei by XENON100, XENON1T, LUX [30,31,32], gamma rays from annihilation of dark matter interpreted by Fermi-LAT data [33, 34], and the theoretical aspects such as perturbativity, vacuum stability, electroweak phase transition, gravitational waves have been investigated
Summary
The strongly first-order phase transition in the early universe is one of the three Sakharov conditions [87] for the electroweak baryogenesis. In the SM framework, a strong first-order phase transition gives the Higgs mass an upper limit, mH 70 GeV [88, 89], which is in conflict with the measured Higgs mass at the LHC being 125 GeV This motivates the extension of the SM which among numerous possible extensions the addition of a real singlet scalar is the simplest. We analyze the model in two parts, once with the s-s cross-coupling terms for the scalars s and s , i.e. s2s 2 and ss 3 and s3s , and once without these s-s cross-coupling terms It can be seen from eq (A.1) that at very high temperature, T → ∞, the only extremum of the thermal effective potential is the point (v = 0, w = 0, w = 0) in VEV space.
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