Electric Dipole Moments in Two-Higgs-Doublet Models (1308.6283v2)

Abstract: Electric dipole moments are extremely sensitive probes for additional sources of CP violation in new physics models. Specifically, they have been argued in the past to exclude new CP-violating phases in two-Higgs-doublet models. Since recently models including such phases have been discussed widely, we revisit the available constraints in the presence of mechanisms which are typically invoked to evade flavour-changing neutral currents. To that aim, we start by assessing the necessary calculations on the hadronic, nuclear and atomic/molecular level, deriving expressions with conservative error estimates. Their phenomenological analysis in the context of two-Higgs-doublet models yields strong constraints, in some cases weakened by a cancellation mechanism among contributions from neutral scalars. While the corresponding parameter combinations do not yet have to be unnaturally small, the constraints are likely to preclude large effects in other CP-violating observables. Nevertheless, the generically expected contributions to electric dipole moments in this class of models lie within the projected sensitivity of the next-generation experiments.

Electric Dipole Moments in Two-Higgs-Doublet Models

This paper provides a comprehensive analysis of electric dipole moments (EDMs) within the framework of Two-Higgs-Doublet Models (2HDMs), particularly focusing on their sensitivity to CP-violating phases. The authors investigate various contributions to EDMs, including charged and neutral Higgs exchanges, and explore the implications for 2HDMs without flavor-changing neutral currents (FCNCs).

Overview of EDM Sensitivity in 2HDMs

EDMs serve as sensitive probes for new sources of CP violation beyond the Standard Model (SM). In the SM, CP violation is intricately tied to flavor violation through the Kobayashi-Maskawa mechanism, resulting in minuscule EDMs far below experimental reach. However, the inclusion of new CP-violating phases in 2HDMs can lead to significant EDM contributions unless suppressed by additional mechanisms.

Two-loop contributions, particularly Barr-Zee diagrams, dominate the EDM landscape in 2HDMs due to the inherent suppression of one-loop contributions by the flavor symmetries that avoid tree-level FCNCs. Charged Higgs exchange contributes notably to the Weinberg operator, while both charged and neutral Higgs exchanges influence quark and electron EDMs.

Charged Higgs and EDM Constraints

Charged Higgs boson exchanges are pivotal in 2HDMs regarding EDM observables. Through two-loop processes, they particularly contribute to the EDMs of neutrons and electrons. For the electron EDM, contributions from Barr-Zee diagrams with charged Higgses impose stringent bounds on CP-violating Yukawa combinations. Similarly, for the neutron EDM, the effective two-loop generation of the Weinberg operator presents significant constraints. These constraints do not necessitate fine-tuning of parameters yet would be challenged by the next generation of EDM experiments.

Neutral Higgs Contributions

The neutral Higgs sector in 2HDMs introduces complexity due to the mixing among the three neutral fields. The orthogonality of the scalar mixing matrix often leads to large cancellations in EDM contributions. Notably, in both universal Higgs mass scenarios and decoupling limits, the sum of contributions mediated by neutral Higgses disappears. To estimate these effects, the authors introduce an average scalar mass for evaluation, taking into account possible cancellations.

Implications for 2HDMs

EDMs provide powerful constraints on CP-violating parameters in 2HDMs, rivaling those from flavor physics observables like the branching ratio of $b \to s \gamma$. The complimentary nature of EDM constraints highlights their critical role in probing NP models. While the current limits are stringent, further improvements in experimental sensitivity are expected to explore parameter space where large effects could potentially be observed.

Potential for Future Developments

The paper highlights the implicit expectation of a rich phenomenological landscape within the sensitivity of near-future experimental setups. EDMs stand as a good predictive tool, exploring areas where traditional probes may not reach, especially when complete models exhibit minimal impact within direct collider searches or flavor-changing processes. Given the current and anticipated enhanced experimental capabilities, a substantive exploration of EDMs in measurements could lead to significant discoveries in particle physics, offering insight both practically in terms of direct observation and theoretically, to refine existing models of NP.

In conclusion, while this paper provides strong constraints on CP-violating phases in 2HDMs, it also projects optimism for the discovery potential in upcoming experiments to further elucidate the underlying principles of NP beyond the SM. As such, the paper serves as a foundational paper for researchers attempting to bridge theoretical predictions with experimental advancements in the quest for understanding CP violation in beyond Standard Model physics.