Two-Higgs-doublet model

The data seems to be matching well with the Standard Model (SM) predictions. However there is a strong belief due to many un-answered questions like the Dark matter, Neutrino masses and mixings, Hierarchy problem, Strong CP-problem that physics beyond SM must exist. Two-Higgs-doublet model (2HDM) 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 example three Higgs doublet models etc. The data seems to be matching well with the Standard Model (SM) predictions. However there is a strong belief due to many un-answered questions like the Dark matter, Neutrino masses and mixings, Hierarchy problem, Strong CP-problem that physics beyond SM must exist. Two-Higgs-doublet model (2HDM) 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 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 h {displaystyle h} and H {displaystyle H} (where H {displaystyle H} is heavier than h {displaystyle h} by convention), the CP odd pseudoscalar A {displaystyle A} and two charged Higgs bosons H ± {displaystyle H^{pm }} . The discovered higgson is measured to be CP even, so one can map either h {displaystyle h} or H {displaystyle H} with the observed Higgs. A special case occurs when cos ⁡ ( β − α ) → 0 {displaystyle cos(eta -alpha ) ightarrow 0} , the alignment limit, in which the lighter CP even Higgs boson h {displaystyle h} has couplings exactly like the SM-Higgs boson. In another limit such limit, where sin ⁡ ( β − α ) → 0 {displaystyle sin(eta -alpha ) ightarrow 0} , the heavier CP even boson i.e. H {displaystyle H} is SM-like, leaving h {displaystyle h} to be the lighter than the discovered Higgs. Such a model can be described in terms of has six physical parameters: four Higgs masses ( m h , m H , m A , m H ± {displaystyle m_{h},m_{H},m_{A},m_{H^{pm }}} ), the ratio of the two vacuum expectation values ( tan ⁡ β {displaystyle an eta } ) and the mixing angle ( α {displaystyle alpha } ) which diagonalizes the mass matrix of the neutral CP even Higgses. SM uses only 2 parameters: higgson mass and its vacuum expectation value. 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 (up-type quarks, down-type quarks and charged leptons) 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 Φ {displaystyle Phi } , one can divide two-Higgs-doublet models into the following classes: By convention, Φ 2 {displaystyle Phi _{2}} is the doublet to which up-type quarks couple.