Two-Higgs-doublet model

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 $\cos(\beta -\alpha )\rightarrow 0$, 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 $\sin(\beta -\alpha )\rightarrow 0$, the heavier CP even boson, i.e. H is SM-like, leaving h to be the lighter than the discovered Higgs; however, it is important to note that experiments have strongly pointed towards a value for $\sin(\beta -\alpha )$ that is close to 1.

Such a model can be described in terms of six physical parameters: four Higgs masses ($m_{\rm {h}},m_{\rm {H}},m_{\rm {A}},m_{\mathrm {H} ^{\pm }}$), the ratio of the two vacuum expectation values ($\tan \beta$) and the mixing angle ($\alpha$) 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 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 $\Phi$, 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	$\Phi _{2}$	$\Phi _{2}$	$\Phi _{2}$	charged fermions only couple to second doublet
Type II	MSSM-like	$\Phi _{2}$	$\Phi _{1}$	$\Phi _{1}$	up- and down-type quarks couple to separate doublets
X	Lepton-specific	$\Phi _{2}$	$\Phi _{2}$	$\Phi _{1}$
Y	Flipped	$\Phi _{2}$	$\Phi _{1}$	$\Phi _{2}$
Type III		$\Phi _{1},\Phi _{2}$	$\Phi _{1},\Phi _{2}$	$\Phi _{1},\Phi _{2}$	Flavor-changing neutral currents at tree level
Type FCNC-free		$\Phi _{1},\Phi _{2}$	$\Phi _{1},\Phi _{2}$	$\Phi _{1},\Phi _{2}$	By finding a matrix pair which can be diagonalized simultaneously.

By convention, $\Phi _{2}$ is the doublet to which up-type quarks couple.

See also

• Alternatives to the Standard Model Higgs
• Composite Higgs models
• Preon