Two-Higgs-doublet model

Now with the recent 2012 famous Higgs boson discovery, the real question to ask is that have we reached the end of the Particle physics or is it just the beginning. Well at present the data seems to be matching quite well with the Standard Model (SM)predictions (by the way SM is a model which has successfully described the particles interactions till date pretty well). So if everything is matching with the SM, does that really mean that we have reached towards the end of the particle physics or well at-least the collider physics. There is a strong belief due to many un-answered questions like the Dark matter, Neutrino masses and mixings, Hierarchy problem, Strong CP-problem that physics beyond SM must exist.

In one such attempt, a two-Higgs-doublet model (2HDM) which is one of the simplest extensions of the Standard Model (SM) is described in this article.^[1] If we don't want to restrict ourselves to the major issues we can argue such models based on the simple philosophy i.e. why we have just one Higgs doublets, in principle there can be many. Therefore 2HDM 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 example three Higgs doublet models etc. However in this page, we shall be confining ourselves to discussion of the 2HDMs.

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 $H^{\pm }$ . Now as we have discovered 125-GeV Higgs Boson at the Large Hadron Collider at both ATLAS and CMS detectors, the immediate question which arises is that which of the scalar amongst these five will correspond to the discovered scalar? Since the discovered Higgs is measured to be CP even, we can map either $h$ or $H$ with the observed Higgs. Attempts have been made in the literature with both the possibilities. A special case occurs when $\cos(\beta -\alpha )\rightarrow 0$ , the so-called alignment limit, in which the lighter CP even Higgs boson $h$ has couplings exactly like the SM-Higgs boson.^[2] 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.

Such a model can be described in terms of has six physical parameters: four Higgs masses ( $m_{h},m_{H},m_{A},m_{{H^{\pm }}}$ ), 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.

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
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. ^[5]

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

References

↑ Gunion, J.; H. E. Haber; G. L. Kane; S. Dawson (1990). The Higgs Hunters Guide. Addison-Wesley.
↑ Craig, N.; Galloway, J.; Thomas, S. (2013). "Searching for Signs of the Second Higgs Doublet". arXiv:1305.2424.
↑ 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.
↑ 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.
↑ https://arxiv.org/pdf/1612.02891v3.pdf

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[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.

[5] ttps://arxiv.org/pdf/1612.02891v3.pdf