Flavor-phenomenology of two-Higgs-doublet models with generic Yukawa structure

Abstract: We perform an extensive study of flavor observables in a 2HDM with generic Yukawa structure (the decoupling limit of the MSSM). We examine the effects in flavor physics and constrain the model both from tree-level processes and loop-observables. The free parameters of the model are the heavy Higgs mass, tan(\beta) and the "non-holomorphic" Yukawa couplings \epsilon^f_ij (f=u,d,\ell). In our analysis we constrain the elements \epsilon^f_ij in various ways: In a first step we give order of magnitude constraints on \epsilon^f_ij from 't Hooft's naturalness criterion. Then we constrain the Yukawa structure of the type-III 2HDM from tree-level FCNC processes (B_s,d->\mu^+\mu^-, \tau^-->\mu^-\mu^+\mu^-, etc.) and observe that all flavor off-diagonal elements except \epsilon^u_32,31 and \epsilon^u_23,13 must be very small in order to satisfy the experimental bounds. In a third step, we consider Higgs mediated loop contributions to FCNC processes (b->s,d\gamma, \mu->e\gamma, etc.) finding that also \epsilon^u_13 and \epsilon^u_23 must be very small, while the bounds on \epsilon^u_31 and \epsilon^u_32 are especially weak. Taking into account the constraints from FCNC processes we study the possible size of contributions from tree-level charged Higgs exchange. Interestingly, the unconstrained elements \epsilon^u_{32,31} enter directly the branching ratios for B->\tau\nu, B->D\tau\nu and B->D*\tau\nu and we show that the deviations from the SM predictions in these processes can be explained simultaneously. We give upper limits on the branching ratios of the lepton flavor-violating neutral B meson decays and correlate the radiative lepton decays to the corresponding neutral current lepton decays (\tau^- ->\mu^-\mu^+\mu^-, \tau^- ->e^-\mu^+\mu^- and \mu^- ->e^-e^+e^-). A detailed appendix contains all relevant information for the considered processes for general scalar-fermion-fermion couplings.

• The paper presents a detailed analysis of 2HDM type III's impact on flavor-changing neutral currents and tauonic B decays.
• The study imposes naturalness and tree-level constraints on non-holomorphic Yukawa couplings to avoid fine-tuning and meet experimental bounds.
• The paper demonstrates that unconstrained Yukawa elements can explain deviations from the Standard Model, guiding future collider experiments.

Overview of Two-Higgs-Doublet Models with Generic Yukawa Structure

The study presented in this paper thoroughly explores the flavor phenomenology of two-Higgs-doublet models (2HDM) with a generic Yukawa structure, specifying a 2HDM of type III. These models are considered the decoupling limit of the Minimal Supersymmetric Standard Model (MSSM) and are known for their extensive flavor phenomenology. They allow for significant effects in Flavour-Changing Neutral Currents (FCNC) and tauonic B decays. The authors investigate the possible effects on flavor physics and impose constraints on the model parameters utilizing tree-level processes and loop observables.

Parameters and Constraints

The study parameterizes the model using the heavy Higgs mass, $\tan\beta$ (the ratio of vacuum expectation values), and the non-holomorphic Yukawa couplings $\epsilon^f_{ij} \,(f=u,d,\ell)$. Following a comprehensive analysis, the paper imposes several constraints:

1. Naturalness Constraints: The order of magnitude constraints on $\epsilon^f_{ij}$ from 't Hooft's naturalness criterion suggests that all $\epsilon^f_{ij}$ must be relatively small unless they involve the third generation. This is due to the necessity of avoiding fine-tuned cancellations that are only supported by a corresponding symmetry.
2. Tree-Level FCNC Constraints: Examining tree-level FCNC processes, the study identifies that all flavor off-diagonal elements of these couplings, except $\epsilon^u_{32,31}$ and $\epsilon^u_{23,13}$, must be minimal to satisfy current experimental limits. Furthermore, constraints from loop contributions to FCNC processes inform that $\epsilon^u_{13}$ and $\epsilon^u_{23}$ should be small, while the bounds on $\epsilon^u_{31}$ and $\epsilon^u_{32}$ are particularly weak.
3. Electric Dipole Moments (EDMs): Additional constraints are inferred from EDM measurements, which impact parameter space at higher accuracy for specific $\epsilon^{u,\ell}_{ij}$ elements.

Implications and Future Directions

The results imply that the unconstrained elements $\epsilon^u_{32,31}$ can substantially contribute to the tauonic B decays like B, B, and B, which may explain deviations from the Standard Model (SM) seen in experiments without fine-tuning. These findings provide an interesting insight into how additional scalar degrees of freedom in the 2HDM could be a plausible explanation for the experimental anomalies observed in semileptonic B decays.

In terms of practical applications, this model foregrounds how the extension of the Higgs sector within the SM family could lead to significant phenomenological implications. Specifically, the model shows potential in addressing the discrepancies evident in semileptonic B decays and tauonic processes. Future work will certainly involve experimentally probing these parameter regions to identify signatures of this Yukawa structure.

Theoretically, these insights extend our understanding of non-standard Higgs interactions, suggesting a rich area for exploration in particle physics. As experimental precision progresses, advanced investigations using collider data will be crucial to further validate the assumptions of the 2HDM type III models.

In conclusion, the paper offers valuable contributions and a comprehensive outlook on analyzing 2HDM with generic Yukawa structures, wherein key insights and constraints on flavor observables are gathered, potentially impacting future research in both theoretical and experimental high energy physics.