Enhancing electroweak baryogenesis: The role of dimension-six operators in the real singlet extension of the Standard Model

This study investigates the electroweak baryogenesis (EWBG) in the real singlet extension of the Standard Model (SM), including dimension-six operators, which couple the Higgs boson to a real singlet scalar field. We show that the electroweak phase transition is a one-step or a two-step phase transition, depending on the parameters of the singlet extension. In the majority of the parameter space, the electroweak phase transition proceeds as a two-step phase transition: an initial phase transition in the singlet sector at high temperatures, followed by a first-order phase transition in the Higgs sector. Interestingly, including the dimension-six operator, the electroweak phase transition occurs in regions of the parameter space, which were ruled out in the singlet extensions of the SM without the dimension-six operator. This is more evident for low singlet masses, which are excluded by the experimental constraints from the invisible decays of the Higgs boson. In addition, the real singlet extension explains the observed baryon asymmetry of the Universe introducing an additional source of CP violation in the SM via a second dimension-six operator, which contributes to the top-quark mass. These results provide new pathways for EWBG, expanding the viable parameter space for future collider and gravitational wave experiments, especially for low singlet masses.