The two-Higgs-doublet model (2HDM) is an extension of the
Standard Model of
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
. 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
The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field,
one of the fields in particle physics theory. In the Stand ...
and (where is heavier than by convention), the CP odd
pseudoscalar
In linear algebra, a pseudoscalar is a quantity that behaves like a scalar, except that it changes sign under a parity inversion while a true scalar does not.
Any scalar product between a pseudovector and an ordinary vector is a pseudoscalar. T ...
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. 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.
Such a model can be described in terms of six physical parameters: four Higgs masses (
), the ratio of the two
vacuum expectation value
In quantum field theory the vacuum expectation value (also called condensate or simply VEV) of an operator is its average or expectation value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle. ...
s (
) 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.
Classification
Two-Higgs-doublet models can introduce
flavor-changing neutral current
In particle physics, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical interactions that change the flavor of a fermion without altering its electric charge.
Details
If they occur in nature (as refl ...
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
, one can divide two-Higgs-doublet models into the following classes:
By convention,
is the doublet to which up-type quarks couple.
See also
*
Alternatives to the Standard Model Higgs
*
Composite Higgs models
*
Preon
In particle physics, preons are point particles, conceived of as sub-components of quarks and leptons.
The word was coined by Jogesh Pati and Abdus Salam, in 1974. Interest in preon models peaked in the 1980s but has slowed, as the Standard Mode ...
References
Physics beyond the Standard Model
Hypothetical composite particles
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