Theory and phenomenology of two-Higgs-doublet models

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.

• The paper outlines the comprehensive Higgs potential and symmetry constraints that mitigate flavor-changing neutral currents.
• It details the phenomenological effects of CP violation and charged Higgs decay modes, highlighting prospects for LHC detection.
• It discusses implications for dark matter candidates and unitarity constraints, guiding future experimental investigations.

Overview of Two-Higgs-Doublet Models (2HDMs)

The study of two-Higgs-doublet models (2HDMs) expands the standard model of particle physics by incorporating an additional scalar doublet. This modification results in a richer phenomenological landscape, characterized by new particles and potential sources of CP violation. The paper provides a comprehensive review of the theoretical and phenomenological aspects of 2HDMs, highlighting key features, symmetry considerations, potential structures, and implications for particle physics.

Theoretical Framework

The 2HDM introduces a second SU(2) scalar doublet, leading to potential new physical phenomena, such as flavor-changing neutral currents (FCNCs) and CP violation. The general Higgs potential in these models involves multiple parameters, including both quadratic and quartic terms:

• Quadratic Terms: These include mass parameters for the doublets, accommodating mass splitting and interactions between the fields.
• Quartic Terms: These include various coupling constants, which describe the interactions between the Higgs fields.

A key focus of 2HDM research is the ability to write the most general Higgs potential in terms of two complex scalar fields, leading to an increase in the parameter space and potential complexity in phenomenological predictions.

Symmetry Constraints

2HDMs often include specific symmetries to mitigate problematic features such as FCNCs:

• Z2 Symmetry: Imposing this symmetry prevents FCNCs by ensuring that each type of fermion couples to only one Higgs doublet.
• CP Symmetry: Several forms of CP symmetry can be imposed to constrain the potential further, leading to simplifications while maintaining predictive power.

These symmetries result in six distinct classes of Higgs potentials within 2HDMs, each characterized by different patterns of scalar mass eigenstates and interactions. The symmetry-driven classification aids in understanding the potential phenomenological consequences of each model.

2HDM Phenomenology

The paper provides an in-depth examination of the phenomenological implications of 2HDMs, including:

• Neutral Scalar Sector: The possibility of CP violation through scalar interactions is a hallmark of 2HDMs, where specific conditions on the potential parameters can lead to spontaneous CP violation.
• Charged Higgs Sector: The presence of charged Higgs bosons introduces new decay modes, influencing processes such as top-quark decays.
• Stability and Unitarity Constraints: Ensuring the boundedness of the scalar potential from below and the unitarity of scattering amplitudes imposes constraints on the model's parameters, affecting the allowed range of scalar masses and couplings.

Implications and Future Directions

2HDMs offer rich avenues for beyond-the-standard-model physics, with implications for collider physics and the understanding of CP violation:

• LHC Searches: The Large Hadron Collider (LHC) can probe the extended Higgs sector by searching for additional scalar particles, such as heavier neutral scalars or charged Higgs bosons.
• Dark Matter Candidate: Certain configurations in the scalar potential, specifically inert doublets, yield stable particles that can act as dark matter candidates.

The 2HDM framework provides a promising platform for exploring new physics scenarios, potentially addressing questions related to the matter-antimatter asymmetry and the nature of dark matter. As experimental searches progress, the constraints and predictions of 2HDMs will continue to evolve, contributing to the broader effort to unravel the mysteries of the Higgs sector and the fundamental forces governing particle interactions.