Theory and phenomenology of two-Higgs-doublet models

TL;DR

The work surveys two-Higgs-doublet models (2HDMs), detailing how discrete and continuous symmetries control flavor-changing neutral currents, the various NFC realizations (Type I/II, Lepton-specific, Flipped, Inert), and the structure of the scalar sector including the potential, vacuum stability, and basis choices. It also analyzes models with tree-level FCNC (Type III, BGL, MFV), charged-Higgs phenomenology, and CP-violation scenarios (explicit and spontaneous) with invariant diagnostics. A substantial portion is devoted to the scalar potential’s symmetry classes, basis-invariant CP tests, and the Higgs-basis formalism, linking theoretical structure to collider signatures. The review connects these theoretical frameworks to LHC-era results, illustrating how current data constrains the 2HDM parameter space and outlining avenues for future exploration in Higgs decays, production channels, and CP-violating observables.

Abstract

We discuss theoretical and phenomenological aspects of two-Higgs-doublet extensions of the Standard Model. In general, these extensions have scalar mediated flavour changing neutral currents which are strongly constrained by experiment. Various strategies are discussed to control these flavour changing scalar currents and their phenomenological consequences are analysed. In particular, scenarios with natural flavour conservation are investigated, including the so-called type I and type II models as well as lepton-specific and inert models. Type III models are then discussed, where scalar flavour changing neutral currents are present at tree level, but are suppressed by either specific ansatze for the Yukawa couplings or by the introduction of family symmetries. We also consider the phenomenology of charged scalars in these models. Next we turn to the role of symmetries in the scalar sector. We discuss the six symmetry-constrained scalar potentials and their extension into the fermion sector. The vacuum structure of the scalar potential is analysed, including a study of the vacuum stability conditions on the potential and its renormalization-group improvement. The stability of the tree level minimum of the scalar potential in connection with electric charge conservation and its behaviour under CP is analysed. The question of CP violation is addressed in detail, including the cases of explicit CP violation and spontaneous CP violation. We present a detailed study of weak basis invariants which are odd under CP. A careful study of spontaneous CP violation is presented, including an analysis of the conditions which have to be satisfied in order for a vacuum to violate CP. We present minimal models of CP violation where the vacuum phase is sufficient to generate a complex CKM matrix, which is at present a requirement for any realistic model of spontaneous CP violation.

Figures (22)

• Figure 1: The branching ratios for the decay of the SM Higgs boson as a function of its mass.
• Figure 2: The type-I 2HDM light-Higgs branching ratios into $W$ pairs, diphotons and $b\bar{b}$ are plotted as a function of $\alpha$ for $\tan\beta=1$ and for various values of the Higgs mass (in GeV). In the left figure, the solid lines correspond to $h\rightarrow WW$ and the dashed lines to $h\rightarrow \gamma\gamma$.
• Figure 3: Branching ratios of the light Higgs boson $h$ into a $Z$ and a pseudoscalar, for various values of the mass (in GeV) of $h$. The value of $\tan\beta$ is chosen to be 1 and the mass of the pseudoscalar is chosen to be 5 GeV.
• Figure 4: Branching ratios of the heavy Higgs boson into $b \bar{b}$ (solid lines) and $t \bar{t}$ (dashed lines), for various values of the heavy-Higgs mass. We have chosen $\tan{\beta} = 1$.
• Figure 5: The type II 2HDM light-Higgs branching ratios into $W$ pairs, diphotons and $b\bar{b}$ are plotted as a function of $\alpha$ for $\tan\beta=1$ and $\tan\beta=6$ and for various values of the Higgs mass (in GeV). In the left-hand figures, the solid lines correspond to $h\rightarrow WW$ and the dashed lines to $h\rightarrow \gamma\gamma$. The branching ratio into $Z$ pairs has the same ratio to the one into $W$ pairs as in the Standard Model.