2HDM

2HDM stands for the two-Higgs-doublet model, an extension of the Standard Model in which the single complex scalar SU(2)L doublet is replaced by two such doublets, Φ1 and Φ2. After electroweak symmetry breaking, the spectrum contains five physical scalars: two CP-even neutral Higgs bosons h and H, a CP-odd neutral pseudoscalar A, and a charged Higgs pair H±. The electroweak scale is v ≈ 246 GeV with tanβ = v2/v1.

To prevent tree-level flavor-changing neutral currents, the model imposes a Z2 symmetry under which Φ1→Φ1, Φ2→-Φ2 (softly broken by m12^2). The scalar potential includes quadratic terms, a soft-breaking term m12^2, and quartic couplings λ1–λ5. The angles α and β parametrize the mixing of neutral states and the ratio of VEVs, with v^2 = v1^2+v2^2.

To specify fermion couplings and avoid FCNCs, different Z2 charge assignments lead to four canonical types: Type I, II, X (lepton-specific), and Y (flipped). In Type II, up-type quarks couple to Φ2, while down-type quarks and leptons couple to Φ1. The other types distribute couplings differently. Depending on parameters, one CP-even state can resemble the SM Higgs (alignment limit) with α ≈ β−π/2.

Phenomenology includes searches for H, A, H± at colliders, modified Higgs decays, and flavor constraints such as b→sγ, which limits the charged Higgs mass in several types. The decoupling limit occurs when extra Higgs states are heavy, leaving a SM-like h. Theoretical considerations require vacuum stability, perturbativity, and unitarity.