Flavor models for $\bar{B} \to D^{(*)} τ\barν$ (1506.08896v4)

Abstract: The ratio of the measured $\bar B\to D^{(*)}\ell\bar{\nu}$ decay rates for $\ell = \tau$ vs. $e,\mu$ deviate from the Standard Model (SM) by about $4\sigma$. We show that the data are in tension with the SM, independent of form factor calculations, and we update the SM prediction for $\mathcal{B}(B\to X_c\tau\bar{\nu}) / \mathcal{B}(B\to X_c \ell\bar{\nu})$. We classify the operators that can accommodate the measured central values, as well as their UV completions. We identify models with leptoquark mediators that are minimally flavor violating in the quark sector, and are minimally flavor violating or $\tau$-aligned in the lepton sector. We explore experimental signatures of these scenarios, which are observable in the future at ATLAS/CMS, LHCb, or Belle II.

• The paper rigorously analyzes B→D(*)τν decay anomalies, revealing approximately 4σ deviations from Standard Model predictions through effective operator fits.
• It employs a detailed operator analysis and χ² minimization to identify viable new physics contributions, notably favoring leptoquark models over type-II 2HDMs.
• The study underlines future experimental prospects at Belle II and LHCb, emphasizing precise measurements of kinematic distributions to further test NP hypotheses.

Analyzing Flavor Models in $\bar{B} \to D^{(*)}\tau\bar{\nu}$ Decays

This paper addresses the persistent anomalies observed in the ratios of decay rates $\bar{B}\to D^{(*)}\tau\bar{\nu}$ versus $\bar{B}\to D^{(*)}\ell\bar{\nu}$ (where $\ell = e, \mu$), which reportedly deviate from the Standard Model (SM) predictions by approximately $4\sigma$. The discussion presented challenges the standard expectations by proposing that these measurements show significant tension with SM predictions regardless of the form factor calculations.

Research Context and Motivation

The $\bar{B} \to D^{(*)}\tau\bar{\nu}$ decays have garnered attention due to their potential sensitivity to new physics beyond the SM, particularly in mediating third-generation fermion interactions. Ordinarily, such decay channels offer rich insights since they are less constrained by flavor physics compared to intergenerational transitions. The nominal phase space restrictions due to $\tau$ mass and absence of CKM or loop suppression (unlike other $B$ decays) make $\bar{B} \to D^{(*)}\tau\bar{\nu}$ a stand-out candidate for unearthing new interactions.

Summary of the Methodology

The authors carry out a rigorous operator analysis to identify possible new physics (NP) contributions that can accommodate the central values of the observed $R(D^{(*)})$. They categorize effective operators which could mediate the $\bar{B} \to D^{(*)}\tau\bar{\nu}$ transition and consider their ultraviolet completions. Leptoquarks prominently feature in these discussions, hypothesized as possible mediators that are minimally flavor violating in both quark and lepton sectors.

Numerical Results and Analysis

The paper includes a comprehensive fit analysis, leveraging $\chi^2$ minimization to constrain operator coefficients subject to current experimental data constraints. Viable operators that withstand the constraints include those resembling the SM vector form but with additional contributions. Theoretical calculations regarding heavy quark expansions play a significant role in establishing bounds on these operator effects. A notable result is that conventional type-II two-Higgs-doublet models (2HDMs) do not conform well to the observed data, leading to their exclusion under specific parameter spaces.

Implications and Future Directions

The explorations imply significant potential for new gauge fields or scalar exchanges, emphasizing the intriguing role of leptoquarks. The paper encourages further experimental research to precisely map the decay distributions, especially focusing on kinematic variables like the dilepton invariant mass $q^2$. Future experiments, particularly at the upgraded Belle~II and continued studies at LHCb, are poised to tighten the uncertainties and test specific NP hypotheses further.

Moreover, the paper suggests that inconsistencies between predicted and observed $R(D^{(*)})$ advocate for a careful reevaluation of NP flavor models, possibly extending beyond the strictures of MFV. Horizontal symmetries, particularly $U(1)_\tau$ in the lepton sector, are also evaluated as complementary avenues to account more naturally for the decay discrepancies without inducing rampant FCNCs or conflicting signals in low-energy observables.

Conclusion

The investigation articulates a strong narrative for a theoretical paradigm shift to address $\bar{B} \to D^{(*)}\tau\bar{\nu}$ decay anomalies. It lays the groundwork for a detailed look at leptoquark models, signaling a need to balance between effective field theories and potential ultraviolet completions that maintain consistency with broader flavor data. As precision measurements evolve, the dialogue between theoretical consistency and experimental evidence will continue guiding the search for possible beloved yet elusive new particles.