two-Higgs-doublet models

two-Higgs-doublet models
Name	Two-Higgs-doublet models
Caption	Schematic of scalar potential in a two-Higgs-doublet model
Field	Particle physics
Introduced	1970s
Notable	Glashow–Weinberg condition, Type I, Type II, CP violation, supersymmetry

two-Higgs-doublet models Two-Higgs-doublet models are extensions of the Standard Model that introduce a second complex scalar doublet, increasing the scalar sector and enabling additional sources of CP violation, flavor structure, and mass generation. Developed in the 1970s alongside work by T. D. Lee and others, these models connect to frameworks such as supersymmetry, grand unification, and electroweak baryogenesis, and they are probed by experiments at facilities like the Large Hadron Collider and experiments conducted by collaborations including ATLAS collaboration and CMS.

Introduction

Two-Higgs-doublet models (2HDMs) extend the Higgs mechanism of the Glashow–Weinberg–Salam electroweak sector by introducing two SU(2)_L scalar doublets, often labeled Φ1 and Φ2. They were motivated historically by attempts to explain CP violation beyond the Kobayashi–Maskawa theory and to provide scalar sectors compatible with supersymmetric models such as the Minimal Supersymmetric Standard Model. Prominent theorists and institutions contributing to 2HDM development include T. D. Lee, Steven Weinberg, and workshops held at places like CERN and SLAC National Accelerator Laboratory.

Theoretical Framework

The 2HDM framework specifies a scalar potential invariant under SU(2)×U(1) electroweak gauge symmetry, with parameters that control mass eigenstates and mixing angles. The scalar spectrum typically contains two CP-even Higgs bosons (often denoted h and H), one CP-odd pseudoscalar A, and a charged Higgs pair H^±; mixing is parametrized by angles commonly labeled α and β. Imposing discrete symmetries such as a Z2 symmetry avoids tree-level flavor-changing neutral currents, a criterion related to the Glashow–Weinberg condition, and connects to models studied by groups at Fermilab and theoretical programs at Institute for Advanced Study. The Higgs potential and Yukawa couplings are constructed to preserve gauge invariance and renormalizability, with loop corrections calculated using techniques developed in the context of quantum field theory by authors associated with Princeton University and Harvard University.

Types and Variants

Common 2HDM types are categorized by how fermions couple to the two doublets. Type I, Type II, Type X (lepton-specific), and Type Y (flipped) are widely used in phenomenology and were discussed in literature from groups at KEK and DESY. The Type II realization is realized in the Minimal Supersymmetric Standard Model and links to research from collaborations at University of Oxford and Cambridge University. Other variants include inert doublet models, used in dark matter studies by teams at Max Planck Institute for Physics, and aligned 2HDMs motivated by flavor symmetry programs at Massachusetts Institute of Technology. Models incorporating explicit or spontaneous CP violation were analyzed by researchers at Argonne National Laboratory and in workshops at Perimeter Institute.

Phenomenology and Signatures

Phenomenological consequences of 2HDMs include modified Higgs couplings affecting production and decay rates measured by ATLAS collaboration and CMS, the presence of charged Higgs bosons observable in top quark decays at Tevatron experiments, and novel CP-violating observables relevant to baryogenesis scenarios discussed at SLAC seminars. Collider signals include H→ZZ, A→Zh, and H^±→τν or tb channels explored by analysis groups at CERN and Fermilab. Flavor observables such as B→X_sγ and Bs mixing provide complementary probes by collaborations like Belle and LHCb. Astrophysical implications arise in inert versions where the lightest neutral scalar can be a dark matter candidate investigated by teams at Gran Sasso National Laboratory and XENON collaborations.

Constraints and Experimental Tests

Experimental constraints derive from Higgs coupling fits by ATLAS collaboration and CMS, precision electroweak measurements by groups at LEP and SLD, and flavor limits from BaBar, Belle, and LHCb. Direct searches set mass bounds on charged Higgses using data from Tevatron and Large Hadron Collider. Global fits incorporate theoretical constraints such as perturbative unitarity, vacuum stability, and oblique parameters S, T, U studied at institutes including CERN and Institut für Kernphysik, Mainz. Future prospects are driven by planned upgrades at HL-LHC and proposed colliders like the International Linear Collider and FCC.

Extensions and Applications

2HDMs serve as building blocks for broader theories: the Minimal Supersymmetric Standard Model contains a Type II 2HDM at tree level, while multi-Higgs scenarios appear in left–right symmetric models and grand unified theories explored by researchers at CERN and Institute of Theoretical Physics, Chinese Academy of Sciences. Applications include electroweak baryogenesis proposals advanced by groups at University of Chicago and dark matter model-building in inert 2HDMs pursued at Kavli Institute for the Physics and Mathematics of the Universe. Flavor model embeddings use flavor symmetries studied by collaborations at Perimeter Institute and University of Cambridge.

Mathematical Formalism and Calculations

The mathematical formalism employs Lagrangian field theory with kinetic terms, a scalar potential V(Φ1,Φ2), and Yukawa interaction matrices that are diagonalized via unitary transformations linked to the Cabibbo–Kobayashi–Maskawa matrix framework developed at Nagoya University and Cornell University. Mass matrices are obtained by minimizing V and diagonalizing the Hessian; loop-level corrections are computed using renormalization techniques and regularization schemes formalized by researchers at CERN and SLAC National Accelerator Laboratory. Perturbative unitarity bounds and vacuum stability criteria are derived from scattering amplitudes evaluated using methods from Institut des Hautes Études Scientifiques and applied numerically in public tools created by collaborations at University of Geneva and University of Zurich.

Category:Beyond the Standard Model