Strongly First Order Phase Transitions and The Possibility of Electroweak Baryogenesis in General Two-Higgs-Doublet Models

University essay from Lunds universitet/Institutionen för astronomi och teoretisk fysik; Lunds universitet/Teoretisk partikelfysik

Abstract: The possibility to have a strongly first order electroweak phase transition, which is one of the Sakharov conditions for electroweak baryogenesis together with $\mathcal{CP}$ violation and baryon number violation, in general Two-Higgs-Doublet Models (2HDMs) is here investigated. Different kinds of $\mathbb{Z}_{2}$ breaking beyond standard assumptions are studied, in particular by allowing it to be completely broken as well as by assuming it to be only approximately preserved in the Yukawa sector. Throughout the study a 1-loop thermally corrected effective potential is used. Parameter space scans are performed for several kinds of 2HDMs and experimental constraints from Higgs physics, flavour physics, electroweak physics and searches for electric dipole moments of the electron, neutron, mercury and radium are imposed. Also theoretical constraints are taken into account, in particular tree level unitarity, perturbativity and positivity as well as demanding that the tree level minimum of the potential is global. As a starting point $\mathcal{CP}$ conserving models are considered, but after that also $\mathcal{CP}$ violating models (which are relevant for electroweak baryogenesis) are investigated. It is found that a reasonable amount of points satisfy all imposed constraints for both $\mathcal{CP}$ conserving and $\mathcal{CP}$ violating models, and that the average strength of a phase transition increases when $\mathcal{CP}$ violation is added, as favoured by the Sakharov conditions. The $\mathcal{CP}$ violating models with only soft $\mathbb{Z}_{2}$ breaking are seen to more easily satisfy the imposed constraints than those with a completely broken $\mathbb{Z}_{2}$ symmetry. In the models with only an approximate $\mathbb{Z}_{2}$ symmetry in the Yukawa sector no difference can be seen when non-diagonal perturbations, as compared to the case of only having diagonal ones, are allowed. It is observed that the electroweak constraints in general require small squared mass differences between the scalar particles, and that the constraints from electric dipole moment searches are the most constraining ones. Finally, it is found that low $\tan \beta$ is generally preferred by all imposed constraints, thus making the picture consistent, and, as is known from the generation of a baryon asymmetry, low $\tan \beta$ is also favoured since the asymmetry $n_{B}\sim \cot ^{2}\beta$. Hence it is concluded that there indeed is hope for 2HDMs to explain the observed baryon asymmetry of the Universe through electroweak baryogenesis.

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