New physics in the weak interaction of $\bar B\to D^{(*)}τ\barν$ (1212.1878v3)

Abstract: Recent experimental results on exclusive semi-tauonic B meson decays, B -> D() tau nu, showing sizable deviations from the standard model prediction, suggest a new physics in which the structure of the relevant weak charged interaction may differ from that of the standard model. We study the exclusive semi-tauonic B decays in a model-independent manner using the most general set of four-Fermi interactions in order to clarify possible structures of the charged current in new physics. It turns out that correlations among observables including tau and D polarizations and q^2 distributions are useful to distinguish possible new physics operators. Further, we investigate some interesting models to exhibit the advantage of our model-independent analysis. As a result, we find that two Higgs doublet models without tree-level FCNC and the minimal supersymmetric standard model with R-parity violation are unlikely to explain the present experimental data, while two Higgs doublet models with FCNC and a leptoquark model are consistent with the data.

• The paper demonstrates that anomalies in semi-tauonic B decays are best explained by new physics operators, notably right-handed currents and tensor interactions.
• It employs a model-independent four-Fermi framework to analyze observables such as tau polarization and q² distributions, detailing the contributions of different operators.
• The study identifies 2HDM with FCNC and leptoquark models as promising candidates, refining the parameter space for theories extending beyond the Standard Model.

New Physics in the Weak Interaction of $\bar B \to D^{(*)} \tau \bar \nu$

The paper by Tanaka and Watanabe presents a comprehensive analysis of the semi-tauonic $B$ meson decays $\bar B \to D^{(*)} \tau \bar \nu$, focusing on potential new physics beyond the Standard Model (SM). Recent experimental data indicating significant deviations from the SM predictions necessitate an examination of the weak interaction structure in these decays.

The paper employs a model-independent framework using the most general four-Fermi interactions to explore the possible charged current structures in new physics scenarios. The authors scrutinize various observables, such as tau and $D^*$ polarizations and $q^2$ distributions, to disentangle potential new physics operators' contributions.

Key Numerical Findings

The paper reveals that existing two-Higgs doublet models (2HDMs) without tree-level flavor-changing neutral currents (FCNC) and the minimal supersymmetric standard model (MSSM) with $R$-parity violation do not adequately describe the observed phenomena. In contrast, the 2HDM with FCNC and leptoquark models align with the experimental data. The crucial insight is that the SM current itself and operators involving right-handed quark currents and tensor interactions are viable in explaining these experimental anomalies.

Specifically, the paper highlights that neither $\mathcal{O}_{S_1}$ nor $\mathcal{O}_{S_2}^{e,\mu}$ satisfactorily explains the current datasets, whereas the operators $\mathcal{O}_{S_2}^{\tau}$, $\mathcal{O}_{V_1}^{l}$, $\mathcal{O}_{V_2}^{l}$, and $\mathcal{O}_T^{l}$ do. This observation is critical as it delimits the parameter space for theorists seeking to model these decay processes with new physics.

Implications and Future Directions

The consistency of the 2HDM with FCNC and leptoquark models suggests promising directions in the exploration of new physics. Notably, the presence of FCNC in the up quark sector could significantly enhance $\mathcal{O}_{S_2}$, aligning with the experimental results. This solicits a reconsideration of 2HDM assumptions, especially concerning the Yukawa sector's symmetry-breaking terms.

Furthermore, the potential role of leptoquarks in generating both $\mathcal{O}_{S_2}$ and $\mathcal{O}_T$ concurrently is suggestive of model classes that could resolve observed discrepancies. This opens a pathway for future model-building efforts to integrate these leptoquark mechanisms.

The methodology outlined for determining new physics contributions emphasizes the utility of combining decay rate measurements, longitudinal tau polarization, and $q^2$ distributions. Future experimental efforts, particularly from super $B$ factories, could leverage this approach to discern the underlying dynamics more precisely. Such data will be pivotal in substantiating or refuting the presence of the proposed new physics operators.

Essentially, the work underscores the nuanced interplay between theoretical predictions and experimental pursuits, advocating for comprehensive data analyses to disentrench the full landscape of charged current interactions. In this context, the paper calls for continued exploration within these frameworks, given the potential for significant discoveries in understanding fundamental interactions beyond the SM.