Two-Higgs-doublet model

In particle physics, a two-Higgs-doublet model (2HDM) is an extension of the Standard Model in which a second Higgs doublet is added.^[1]

The addition of the second Higgs doublet leads to five physical states: 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 $H^\pm$. Such a model has six free parameters: four Higgs masses ($m_h, m_H, m_A, m_{H^\pm}$), the ratio of the two vacuum expectation values ($\tan \beta$) and a mixing angle ($\alpha$).

A special case is when $\cos(\beta - \alpha) = 0$, the so-called alignment limit, in which the lighter CP even Higgs boson $h$ has couplings like the Higgs boson of the Standard Model.^[2]

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:^[3][4]

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

By convention, $\Phi_2$ is the doublet to which up-type quarks couple.

References

1. ↑ Gunion, J.; H. E. Haber; G. L. Kane; S. Dawson (1990). The Higgs Hunters Guide. Addison-Wesley.
2. ↑ Craig, N.; Galloway, J.; Thomas, S. (2013). "Searching for Signs of the Second Higgs Doublet". arXiv:1305.2424.
3. ↑ Craig, N.; Thomas, S. (2012). "Exclusive Signals of an Extended Higgs Sector". Journal of High Energy Physics 1211: 083. arXiv:1207.4835. Bibcode:2012JHEP...11..083C. doi:10.1007/JHEP11(2012)083.
4. ↑ Branco, G. C.; Ferreira, P.M.; Lavoura, L.; Rebelo, M.N.; Sher, Marc; Silva, João P. (July 2012). "Theory and phenomenology of two-Higgs-doublet models". Physics Reports (Elsevier) 516 (1): 1–102. arXiv:1106.0034. Bibcode:2012PhR...516....1B. doi:10.1016/j.physrep.2012.02.002.