Testing leptoquark models in $\bar B \to D^{(*)} τ\barν$ (1309.0301v3)

Abstract: We study potential New Physics effects in the $\bar B \to D^{(*)} \tau \bar\nu$ decays. As a particular example of New Physics models we consider the class of leptoquark models and put the constraints on the leptoquark couplings using the recently measured ratios $R(D^{(*)})=BR(\bar B \to D^{(*)} \tau \bar\nu)/BR(\bar B \to D^{(*)} \mu \bar\nu)$. For consistency, some of the constraints are compared with the ones coming from the current experimental bound on $BR(B \to X_s \nu \bar\nu)$. In order to discriminate various New Physics scenarios, we examine the correlations between different observables that can be measured in future.

An Analysis of Leptoquark Models in $B \to D^{(*)} \tau$ Decays

The paper by Sakaki et al. explores possible implications of New Physics (NP) in the semitauonic decay processes of $B$ mesons, specifically focusing on leptoquark models. This research is catalyzed by reported discrepancies between Standard Model (SM) predictions and experimental results from BaBar and Belle concerning the $R(D^{(*)})$ ratios, defined as $$R(D^{(*)})=B(B \to D^{(*)} \tau)/B(B \to D^{(*)} \mu)$$. These decays are significant because deviations from expected SM outcomes provide potential windows to explore NP theories such as leptoquark models.

Exploration of Leptoquark Models

Leptoquarks, hypothetical particles that connect quarks and leptons, are theorized in several extensions of the SM, given their capability to address unsolved inconsistencies like the SM’s failure to incorporate gravity and unify forces. Specifically, leptoquarks could account for observed anomalies in semileptonic $B$ decays. The researchers examine different classes of scalar and vector leptoquarks, describing their interaction Lagrangians and how these can be constrained using data from $B$ meson decays.

Methodological Framework

The paper uses effective Hamiltonian theory to relate various four-fermion interactions typified in these decay processes. The effectiveness of leptoquarks is evaluated through Wilson coefficients corresponding to vector, scalar, and tensor operators, which encode the strengths of these interactions. Notably, the paper extends previous analyses by exploring all feasible leptoquark models contributing to these decay channels, with a focus on the NP scenarios that align well with experimental discrepancies.

The researchers employ constraints obtained from both $B \to D^{(*)} \tau$ and $B \to X_s \nu$ decays, utilizing a careful treatment of theoretical uncertainties inherent in hadronic form factors. Two sets of form factors, the HQET and MS models, provide the necessary calculus for determining the NP contributions contrasted against empirical data.

Numerical Results and Constraints

The effective couplings in leptoquark models are subjected to a chi-square fit with the experimental $R(D^{(*)})$ values to ascertain their viability. Notably, scalar leptoquarks with significant couplings, like $S_1^{1/3}$ and $R_2^{2/3}$, are shown to adequately account for observed data anomalies. These models predict significant Wilson coefficients ratios, such as $C_{S_2}^l=\mp 4 C_T^l$, that could measurably deviate from SM predictions, offering potentially observable effects in future experiments.

Implications and Future Prospects

The work charts a path for distinguishing different NP models via potential observables such as $\tau$ polarization and $D^*$ polarization in $B \to D^{(*)} \tau$ decays. These observables provide measurable outcomes distinct to each leptoquark scenario, and superior determination at forthcoming colliders could substantiate or refute current predictions.

Sakaki et al., by cross-referencing several data points and leveraging specific operators' sensitivities to hadronic form factor variances, underscore a need for precise theoretical and empirical scrutiny in testing leptoquark models. The analysis suggests pathways for emerging colliders like LHCb and future $B$ factories to probe these avenues, rendering leptoquark detection a realistic prospect beyond theoretical postulate.

Conclusion

This investigation provides robust scrutiny of leptoquark models' efficacy in addressing $B \to D^{(*)} \tau$ decay anomalies. By navigating between theoretical constructs and empirical data, Sakaki et al. contribute a methodical framework that highlights both the challenges and the potential breakthroughs in understanding heavy meson decay processes through the NP lens, maintaining a conscious balance between SM predictions and potential leptoquark presence. Such research enriches our comprehension of particle interactions and delineates a trajectory for NP exploration within quantum field theories.