The two-Higgs-doublet model (2HDM) 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 (where H is heavier than h by convention), 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 ( β − α ) → 0 {\displaystyle \cos(\beta -\alpha )\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 ( β − α ) → 0 {\displaystyle \sin(\beta -\alpha )\rightarrow 0} , the heavier CP even boson, i.e. H is SM-like, leaving h to be the lighter than the discovered Higgs; however, experiments have strongly pointed towards a value for sin ( β − α ) {\displaystyle \sin(\beta -\alpha )} that is close to 1. Such a model can be described in terms of six physical parameters: four Higgs masses ( m h , m H , m A , m H ± {\displaystyle m_{\rm {h}},m_{\rm {H}},m_{\rm {A}},m_{\mathrm {H} ^{\pm }}} ), the ratio of the two vacuum expectation values ( tan β {\displaystyle \tan \beta } ) and the mixing angle ( α {\displaystyle \alpha } ) 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. The masses of the H and A bosons could be below 1 TeV and the CMS experiment has conducted searches around this range but no significant excess above the standard model prediction has been observed.
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 (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.
See also Alternatives to the Standard Model Higgs Composite Higgs models Preon
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