View source for Two-Higgs-doublet model

The '''two-Higgs-doublet model''' ('''2HDM''') is an extension of the [[Standard Model]] of [[particle physics]].<ref>[https://thesis.library.caltech.edu/4505/ "Higgs Scalars and the Nonleptonic Weak Interactions"],  [[Christopher T. Hill]], (1977); see pg. 100.</ref><ref>{{cite book| last1=Gunion| first1=J.|author2=H. E. Haber |author3=G. L. Kane |author4=S. Dawson |title=The Higgs Hunters Guide| publisher=Addison-Wesley |year=1990}}</ref> 2HDM models are one of the natural choices for beyond-SM models containing two [[Higgs boson|Higgs]] doublets instead of just one. There are also models with more than two Higgs doublets, for example three-Higgs-doublet models etc.<ref>{{Cite journal|last1=Keus|first1=Venus|last2=King|first2=Stephen F.|last3=Moretti|first3=Stefano|date=2014-01-13|title=Three-Higgs-doublet models: symmetries, potentials and Higgs boson masses|journal=Journal of High Energy Physics|language=en|volume=2014|issue=1|pages=52|doi=10.1007/JHEP01(2014)052|issn=1029-8479|bibcode=2014JHEP...01..052K|arxiv=1310.8253|s2cid=118482476}}</ref>
The addition of the second Higgs doublet leads to a richer phenomenology as there are five physical scalar states viz., the [[CP symmetry|CP]] even neutral [[Higgs boson|Higgs bosons]] {{Math|h}} and {{Math|H}} (where {{Math|H}} is heavier than {{Math|h}} by convention), the CP odd [[pseudoscalar]] {{Math|A}} and two charged Higgs bosons {{Math|H<sup>±</sup>}}. The discovered Higgs boson is measured to be [[CP symmetry|CP]] even, so one can map either {{Math|h}} or {{Math|H}} with the observed Higgs. A special case occurs  when <math>\cos(\beta - \alpha) \rightarrow  0</math>, the alignment limit, in which the lighter CP even Higgs boson {{Math|h}} has couplings exactly like the SM-Higgs boson.<ref>{{cite arXiv|eprint=1305.2424|first1=N.|last1=Craig|first2=J.|last2=Galloway|title=Searching for Signs of the Second Higgs Doublet|last3=Thomas|first3=S.|year=2013|class=hep-ph}}</ref> In another limit such limit, where <math>\sin(\beta - \alpha) \rightarrow 0</math>, the heavier CP even boson, i.e. {{Math|H}} is SM-like,  leaving {{Math|h}} to be the lighter than the discovered Higgs; however, it is important to note that experiments have strongly pointed towards a value for <math>\sin(\beta - \alpha)</math> that is close to 1.<ref>{{cite journal|arxiv=1809.10733|first1=CMS|last1=Collaboration|title=Combined measurements of Higgs boson couplings in proton–proton collisions at √s=13 TeV |journal=The European Physical Journal C |year=2019|volume=79 |issue=5 |page=421 |doi=10.1140/epjc/s10052-019-6909-y |pmid=31178657 |pmc=6528832 }}</ref>
Such a model can be described in terms of six physical parameters: four Higgs masses (<math>m_{\rm h}, m_{\rm H}, m_{\rm A}, m_{\mathrm{H}^\pm}</math>), the ratio of the two [[vacuum expectation value]]s (<math>\tan \beta</math>) and the mixing angle (<math>\alpha</math>) 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 [[Compact Muon Solenoid|CMS]] has conducted searches around this range but no significant excess above the standard model prediction has been observed.<ref>{{Cite web |title=Hunting the Higgs boson siblings with top quarks {{!}} CMS Experiment |url=https://cms.cern/news/hunting-higgs-boson-siblings-top-quarks |access-date=2023-09-02 |website=cms.cern}}</ref><ref>{{Cite web |title=CMS-PAS-TOP-22-010 |url=https://cms-results.web.cern.ch/cms-results/public-results/preliminary-results/TOP-22-010/index.html |access-date=2023-09-02 |website=cms-results.web.cern.ch |language=en-US}}</ref>
== Classification ==
Two-Higgs-doublet models can introduce [[flavor-changing neutral current]]s 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 <math>\Phi</math>, one can divide two-Higgs-doublet models into the following classes:<ref>{{cite journal
|last1=Craig
|first1=N.
|last2=Thomas
|first2=S.
|year=2012
|title=Exclusive Signals of an Extended Higgs Sector
|journal=[[Journal of High Energy Physics]]
|volume=1211 |issue=11
|pages=083
|doi=10.1007/JHEP11(2012)083
|arxiv = 1207.4835
|bibcode = 2012JHEP...11..083C |s2cid=119312000
}}</ref><ref>{{cite journal
| last1 = Branco
| first1 = G. C.
| title = Theory and phenomenology of two-Higgs-doublet models
| journal = Physics Reports
| volume = 516
| issue = 1
| pages = 1–102
| publisher = Elsevier
| date = July 2012
| doi = 10.1016/j.physrep.2012.02.002
|bibcode = 2012PhR...516....1B |arxiv = 1106.0034
| last2 = Ferreira
| first2 = P.M.
| last3 = Lavoura
| first3 = L.
| last4 = Rebelo
| first4 = M.N.
| last5 = Sher
| first5 = Marc
| last6 = Silva
| first6 = João P.
| s2cid = 119214990
}}</ref>
{| class="wikitable"
! Type
! Description
! up-type quarks couple to
! down-type quarks couple to
! charged leptons couple to
! remarks
|-
| Type I
| Fermiophobic
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_2</math>
| charged fermions only couple to second doublet
|-
| Type II
| [[Minimal Supersymmetric Standard Model|MSSM]]-like
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_1</math>
| style="text-align:center;" | <math>\Phi_1</math>
| up- and down-type quarks couple to separate doublets
|-
| X
| Lepton-specific
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_1</math>
|
|-
| Y
| Flipped
| style="text-align:center;" | <math>\Phi_2</math>
| style="text-align:center;" | <math>\Phi_1</math>
| style="text-align:center;" | <math>\Phi_2</math>
|
|-
| Type III
|
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| [[Flavor-changing neutral current]]s at tree level
|-
| Type FCNC-free
|
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| style="text-align:center;" | <math>\Phi_1, \Phi_2</math>
| By finding a matrix pair which can be diagonalized simultaneously. <ref>{{Cite journal|last1=Botella|first1=Francisco J.|last2=Cornet-Gomez|first2=Fernando|last3=Nebot|first3=Miguel|date=2018-08-30|title=Flavor conservation in two-Higgs-doublet models|journal=Physical Review D|language=en|volume=98|issue=3|pages=035046|doi=10.1103/PhysRevD.98.035046|arxiv=1803.08521 |bibcode=2018PhRvD..98c5046B |issn=2470-0010|doi-access=free}}</ref>
|}
By convention, <math>\Phi_2</math> is the doublet to which up-type quarks couple.
== See also ==
* [[Alternatives to the Standard Higgs Model|Alternatives to the Standard Model Higgs]]
* [[Composite Higgs models]]
* [[Preon]]
==References==
{{reflist|25em}}
[[Category:Physics beyond the Standard Model]]
[[Category:Hypothetical composite particles]]
{{particle-stub}}