Symmetries of 2HDM, different vacua, CP violation and possible relation to a history of time

Abstract

The same physical reality in Two Higgs doublet model (2HDM) can be described by different Lagrangians. We call this property the reparametrization invariance (in space of Lagrangians) and study corresponding symmetry group and its subgroup describing rephasing invariance. Next we consider the $Z_2$-symmetry of the Lagrangian, which prevents a $\phi_1\leftrightarrow\phi_2$ transitions, and the different levels of its violation, soft and hard. We argue that the 2HDM with a soft breaking of $Z_2$-symmetry is a natural model in the description of EWSB. We also consider vacuum structure of the 2HDM. We find very simple condition for a CP violation in the Higgs sector. In the Model II for Yukawa interactions we obtain the set of relations among the couplings to gauge bosons and to fermions which allows one to analyze different physical situations (including CP violation) in terms of these very couplings, instead of the parameters of Lagrangian. We discuss possible interaction of Higgs fields of SM or 2HDM with inflatory Higgs field describing exponential expansion of Universe after Big Bang and possible variation in the scenario of beginning of Time.