Yukawa Alignment in the Two-Higgs-Doublet Model

Abstract: In multi-Higgs-doublet models the alignment in flavour space of the relevant Yukawa matrices guarantees the absence of tree-level flavour-changing couplings of the neutral scalar fields. We analyze the consequences of this condition within the two-Higgs-doublet model and show that it leads to a generic Yukawa structure which contains as particular cases all known specific implementations of the model based on Z_2 symmetries. All possible freedom in the Yukawa sector gets parametrized in terms of three complex couplings. In spite of having flavour conservation in the neutral scalar couplings, the phases of these three parameters represent potential new sources of CP violation.

• The paper introduces a Yukawa alignment mechanism that prevents tree-level FCNCs by parameterizing Yukawa matrices with complex coupling constants.
• It reproduces Z₂-symmetric THDM variants like Type I and II while allowing additional CP-violating phases absent in traditional models.
• Quantum loop corrections remain suppressed, enabling extensive exploration of CP symmetry breaking and potential dark matter signatures.

An Analysis of Yukawa Alignment in the Two-Higgs-Doublet Model

The Two-Higgs-Doublet Model (THDM) often serves as a minimal extension to the Standard Model (SM) by incorporating an additional scalar doublet. This augmentation is a convenient framework to explore low-energy consequences derived from various high-energy physics paradigms, such as supersymmetry. The extension leads to a richer scalar spectrum including two charged and three neutral scalar fields. Notably, the THDM can introduce new sources of CP symmetry breaking, which are crucial for CP violation studies and could have implications for dark matter phenomenology.

A central concern in the THDM is the potential for flavor-changing neutral currents (FCNCs) at the tree level, which are tightly constrained by experimental data. To mitigate this issue, researchers have conventionally adopted discrete Z₂ symmetries, such as the Type I and Type II models. These impose specific couplings between the scalar fields and right-handed fermions, effectively eliminating tree-level FCNCs. Nonetheless, these symmetries restrict the model's phenomenological versatility because they impede additional CP-violating phases apart from the standard ones.

Here, Pich and Tuzón introduce an advanced concept wherein Yukawa alignment is employed to align the flavor space of the Yukawa matrices themselves. This approach eradicates tree-level FCNC interactions without the need for Z₂ symmetry constraints. Instead, the freedom in the Yukawa sector is parameterized by three complex coupling constants, ξf. Although this configuration preserves flavor conservation in neutral scalar interactions, the phases of these parameters represent new potential sources of CP violation.

Key Technical Insights

1. Yukawa Alignment Mechanism: By ensuring alignment in flavor space, the aligned THDM allows for universal Yukawa couplings across different fermion generations while preventing tree-level FCNC interactions. This decoupling is achieved by parameterizing the Yukawa matrices with complex couplings ξf, which are independent of any scalar basis choice.
2. Classification of THDM Types: Specific values for ξf reproduce traditional Z₂-symmetric models, such as:
   • Type I: ξf = cotβ for all fermions.
   • Type II: ξd,l = -tanβ and ξu = cotβ.
   • Type X and Type Y: permutations affecting how couplings depend on tanβ.
3. Loop-Induced Effects: Quantum corrections in the aligned THDM can potentially lead to FCNC effects; however, the correction terms are strictly suppressed due to the inherently structured nature of the quark-mixing matrix and the constrained loop diagrams.
4. CP-Violating Phases: The parameters ξf allow for new CP-violating phenomena which are absent from models constrained by Z₂ symmetries. These effects can be probed in high-energy processes like b → sγ or in the CP asymmetry of neutral meson systems.

Phenomenological Implications

The aligned THDM presents a flexible and inclusive framework for investigating the scalar sector's contribution to flavor physics. Its novel structure allows for an exploration of CP-violating mechanisms and dark matter candidates without introducing FCNCs at tree level. Given the arbitrary nature of the ξf parameters, the model supports a vast parameter space with rich phenomenological potential. This includes multiple mass regimes for the charged scalar H±, explored through decay modes and pair production in electron-positron collisions.

Future Directions

Future research may explore the detailed phenomenological landscape of the aligned THDM, focusing on the identification of CP-violating effects detectable in current and future collider experiments. Moreover, the effects of quantum corrections and their implications for scalar boson mass measurements, Higgs mixing angles, and Yukawa coupling strengths merit comprehensive exploration. The investigation of these aspects could yield additional insights into the nature of electroweak symmetry breaking and the possible realization of the THDM at high energies.

In conclusion, this paper marks a significant development in THDM research by expanding the model's capabilities through Yukawa alignment, offering a platform to explore CP-violating dynamics unfettered by traditional model constraints. This concept not only supplements theoretical predictions but also aligns well with ongoing experimental searches for new physics beyond the Standard Model.