Abstract
Adding an extra singlet scalar $S$ to the Higgs sector can provide a barrier at tree level between a false vacuum with restored electroweak symmetry and the true one. This has been demonstrated to readily give a strong phase transition as required for electroweak baryogenesis. We show that with the addition of a fermionic dark matter particle $\chi$ coupling to $S$, a simple UV-complete model can realize successful electroweak baryogenesis. The dark matter gets a CP asymmetry that is transferred to the standard model through a $CP\ portal\ interaction$, which we take to be a coupling of $\chi$ to $\tau$ leptons and an inert Higgs doublet. The CP asymmetry induced in left-handed $\tau$ leptons biases sphalerons to produce the baryon asymmetry. The model has promising discovery potential at the LHC, while robustly providing a large enough baryon asymmetry and correct dark matter relic density with reasonable values of the couplings.
Highlights
Electroweak baryogensis (EWBG) is the most experimentally testable mechanism for explaining the baryon asymmetry of the Universe (BAU), and as such it is coming under increasing pressure from LHC constraints on new physics below the TeV scale
In a random scan over 670,000 such models, with a flat prior on the intervals (31) we find 600 examples with a strong enough phase transition, and a BAU roughly within an order of magnitude of the required value
We do not claim that this is the only region of parameter space that is viable, nor do we attach any rigorous meaning to the statistics; rather our aim in this study is to demonstrate the existence of one such region, and to establish that it is not the result of any special fine-tuning of parameters
Summary
Electroweak baryogensis (EWBG) is the most experimentally testable mechanism for explaining the baryon asymmetry of the Universe (BAU), and as such it is coming under increasing pressure from LHC constraints on new physics below the TeV scale. A Z2 symmetry is imposed so that χ can couple to standard model leptons, for example the τ lepton doublet Lτ, in combination with an inert Higgs doublet φ, through the interaction yLτφχ We dub this a “CP portal interaction,” since its purpose is to transmit the CP asymmetry between the two helicity states of χ, that is, generated at the bubble wall during the firstorder electroweak phase transition, to Lτ by (inverse) decays of φ. Here the dark matter plays an essential role in generating the baryon asymmetry rather than strengthening the phase transition We show it can be accomplished without any explicit CP violation in the dark sector, since CP can be violated spontaneously in the DM interactions with the scalar singlet in the bubble wall, where S gets a vacuum expectation value (VEV). Tn ∼ 100 GeV, and the new particles are not too Boltzmann suppressed to yield efficient baryogenesis
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