Phenomenology of the basis-independent CP-violating Two-Higgs Doublet Model

The Two-Higgs Doublet Model (2HDM) is a model of low-energy particle interactions that is identical to the Standard Model except for the addition of an extra Higgs doublet. This extended Higgs sector would appear in experiments as the presence of multiple Higgs particles, both neutral and charged. The neutral states may either be eigenstates of CP (in the CP-conserving 2HDM), or be mixtures of CP eigenstates (in the CP-violating 2HDM). In order to understand how to measure the couplings of these new particles, this document presents the theory of the CP-violating 2HDM in a basis-independent formalism and explicitly identifies the physical parameters of the model, including a discussion of tan beta-like parameters. The CP-conserving limit, decoupling limit, and the custodial limit of the model are presented. In addition, phenomenological constraints from the oblique parameters (S, T, and U) are discussed. A survey of the parameter space of this model shows that the 2HDM is consistent with a large range of possible values for T. Our results also suggest that the 2HDM favors a slightly positive value of S and a value of U within .02 of zero, which is consistent with present data within the statistical error. In a scenario in which the heaviest scalar particle is the charged Higgs boson, we find that the measured value of T puts an upper limit on the mass difference between the charged Higgs boson and the heaviest neutral Higgs boson.