twoHiggsdoublet

The two-Higgs-doublet model (2HDM) is an extension of the Standard Model of particle physics that postulates the existence of two scalar Higgs fields instead of just one. In the Standard Model, a single Higgs field is responsible for giving mass to fundamental particles. The 2HDM proposes that two such fields are present, each with its own vacuum expectation value, leading to a richer Higgs sector.

This extension results in a more complex spectrum of Higgs bosons. Instead of a single neutral Higgs boson, the 2HDM typically predicts five Higgs bosons: two CP-even neutral scalars (h and H), one CP-odd neutral pseudoscalar (A), and a pair of charged Higgs bosons (H+ and H-). The interactions of these Higgs bosons with fermions and other Standard Model particles are modified compared to the single-Higgs scenario.

The specific properties and interactions of these Higgs bosons depend on the chosen 2HDM model, which is characterized by different ways the two Higgs doublets couple to fermions. These couplings can be aligned, misaligned, or involve specific structures that lead to different phenomenological consequences. For instance, certain 2HDMs can suppress or enhance flavor-changing neutral currents, which are highly constrained by experimental data.

The 2HDM offers potential solutions to certain problems not addressed by the Standard Model, such as the hierarchy problem (the large difference between the electroweak scale and the Planck scale) and the origin of neutrino masses. Searches for additional Higgs bosons beyond the Standard Model Higgs boson discovered in 2012 are ongoing at particle accelerators like the Large Hadron Collider (LHC), and these searches are sensitive to the predictions of various 2HDM scenarios. Observing such additional Higgs bosons would provide direct evidence for physics beyond the Standard Model.