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The two-Higgs-doublet model is an extension of the Standard Model of particle physics. 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 h and H , the CP odd pseudoscalar A and two charged Higgs bosons H. The discovered Higgs boson is measured to be CP even, so one can map either h or H with the observed Higgs. A special case occurs when cos → 0 {\displaystyle \cos\rightarrow 0} , the alignment limit, in which the lighter CP even Higgs boson h has couplings exactly like the SM-Higgs boson. In another limit such limit, where sin → 0 {\displaystyle \sin\rightarrow 0} , the heavier CP even boson, i.e. H is SM-like, leaving h to be the lighter than the discovered Higgs.
Such a model can be described in terms of 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.