Abstract
Although the Higgs potential in the Standard Model (SM) contains only a simple electroweak symmetry breaking vacuum in the small field region, additional metastable or global vacua could exist in models beyond the SM. In this paper, we study one intriguing scenario with an additional electroweak symmetric vacuum that could be the global one. For the thermal universe ending at the current metastable vacuum, the electroweak symmetry should stay non-restored at high temperatures. We realize the scenario in a model with Higgs-portal couplings to SM singlet scalars with approximately global O(N) symmetries with a large N. For a large portion of model parameter space, both the quantum and thermal tunneling rates are suppressed such that our current metastable vacuum is long-lived enough. Our scenario predicts order-one changes for the Higgs self-couplings and a large contribution to the signal of the off-shell Higgs invisible decay. It can be partly probed at the LHC Run 3 and well tested at the high luminosity LHC. We also discuss the subcritical (anti-de Sitter) bubbles from the thermal tunneling that could have a large population and interesting cosmological implications.
Highlights
The cosmological evolution of the EW vacuum/vacua and the electroweak phase transition (EWPT)
For the thermal universe ending at the current metastable vacuum, the electroweak symmetry should stay non-restored at high temperatures
We realize the scenario in a model with Higgs-portal couplings to Standard Model (SM) singlet scalars with approximately global O(N ) symmetries with a large N
Summary
The scenario considered in this paper is the extension of the SM by including two copies of SM gauge-singlet real scalar fields, S and Φ, transforming as fundamental representations under O(Ns) and O(Nφ), respectively, and coupling to the SM via the Higgs-portal couplings. We will only consider the parameter space with the SM Higgs developing a non-zero VEV h = v = 246 GeV for the vacuum of our current universe sitting on and zero VEVs for both S and Φ At tree level, this requires μ2h > 0, m2s + λhsv2 > 0 and m2φ + λhφv2 > 0.3 In order to prevent the EW symmetry from being restored at high temperatures, we will take λhs < 0 and positive values for other quartic couplings: λh > 0, λs > 0, λφ > 0, λhφ > 0. Requiring the potential is bounded from below with λhs < 0, additional constraints on the quartic couplings can be obtained by focusing on the large field region. As we will discuss below, the experimental probe for the existence of the metastable unbroken vacuum at the origin in our scenarios will be challenging even at the HL-LHC
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