Two-Higgs-doublet model
Currently, experimental data seems to match well with the predictions of the Standard Model. However, there is a widespread belief that due to many unanswered questions like dark matter, neutrino masses, the hierarchy problem, and the strong CP-problem, physics beyond the SM must exist. The Two-Higgs-doublet model is one of the simplest extensions of the SM. 2HDM models are one of the natural choices for beyond-SM models containing two Higgs doublets instead of just one. There are also models with more than two Higgs doublets, for example three Higgs doublet models etc.
The addition of the second Higgs doublet leads to a richer phenomenology as there are five physical scalar states viz., the CP even neutral Higgs bosons and , the CP odd pseudoscalar and two charged Higgs bosons. The discovered Higgs boson is measured to be CP even, so one can map either or with the observed Higgs. A special case occurs when, the alignment limit, in which the lighter CP even Higgs boson has couplings exactly like the SM-Higgs boson. In another limit such limit, where, the heavier CP even boson, i.e. is SM-like, leaving to be the lighter than the discovered Higgs.
Such a model can be described in terms of has six physical parameters: four Higgs masses, the ratio of the two vacuum expectation values and the mixing angle which diagonalizes the mass matrix of the neutral CP even Higgses. The SM uses only 2 parameters: the mass of the Higgs and its vacuum expectation value.
Classification
Two-Higgs-doublet models can introduce Flavor-changing neutral currents which have not been observed so far. The Glashow-Weinberg condition, requiring that each group of fermions couples exactly to one of the two doublets, is sufficient to avoid the prediction of flavor-changing neutral currents.Depending on which type of fermions couples to which doublet, one can divide two-Higgs-doublet models into the following classes:
| Type | Description | up-type quarks couple to | down-type quarks couple to | charged leptons couple to | remarks | - |
| Type I | Fermiophobic | charged fermions only couple to second doublet | - | |||
| Type II | MSSM-like | up- and down-type quarks couple to separate doublets | - | |||
| X | Lepton-specific | - | ||||
| Y | Flipped | - | ||||
| Type III | Flavor-changing neutral currents at tree level | |||||
| Type FCNC-free | By finding a matrix pair which can be diagonalized simultaneously. | - |
By convention, is the doublet to which up-type quarks couple.