The two-Higgs-doublet model (2HDM) is an extension of the Standard Model of particle physics."Higgs Scalars and the Nonleptonic Weak Interactions", Christopher T. Hill, (1977); see pg. 100. 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 (where is heavier than by convention), 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.
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.
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; however, experiments have strongly pointed towards a value for that is close to 1.
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 , 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.
See also
Alternatives to the Standard Model Higgs Composite Higgs models Preon
References