Evidence for an excess of B -> D(*) Tau Nu decays

Abstract: Based on the full BaBar data sample, we report improved measurements of the ratios R(D()) = B(B -> D() Tau Nu)/B(B -> D() l Nu), where l is either e or mu. These ratios are sensitive to new physics contributions in the form of a charged Higgs boson. We measure R(D) = 0.440 +- 0.058 +- 0.042 and R(D) = 0.332 +- 0.024 +- 0.018, which exceed the Standard Model expectations by 2.0 sigma and 2.7 sigma, respectively. Taken together, our results disagree with these expectations at the 3.4 sigma level. This excess cannot be explained by a charged Higgs boson in the type II two-Higgs-doublet model. We also report the observation of the decay B -> D Tau Nu, with a significance of 6.8 sigma.

• The paper presents precise measurements of R(D) = 0.440 ± 0.058 (stat.) ± 0.042 (syst.) and R(D*) = 0.332 ± 0.024 (stat.) ± 0.018 (syst.) in semileptonic B decays.
• The findings diverge from Standard Model expectations by 2.0σ and 2.7σ for R(D) and R(D*), cumulatively reaching a 3.4σ discrepancy.
• These results exclude a charged Higgs in the type II Two-Higgs-Doublet Model, prompting further exploration of potential new physics contributions.

Overview of Semileptonic Decays in B Mesons

The paper presents an analysis of semileptonic decay processes in B mesons, specifically measuring the branching fraction ratios $R(D)$ and $R(D^*)$. These ratios are defined as $$R(D^{(*)}) = \mathcal{B}(B \to D^{(*)} \tau \nu) / \mathcal{B}(B \to D^{(*)} \ell \nu)$$, where $\ell$ denotes the lighter leptons, either electron ($e$) or muon ($\mu$). The measurements are extracted from a comprehensive data set collected with the $\text{B}\text{A}\mathbb{B}\text{AR}$ detector, amounting to a significant $471\times 10^6$ $B\overline{B}$ pairs, recorded at the $\FourS$ resonance.

Key Results

Notable findings include:

• $$R(D) = 0.440 \pm 0.058 \text{(stat.)} \pm 0.042 \text{(syst.)}$$
• $$R(D^*) = 0.332 \pm 0.024 \text{(stat.)} \pm 0.018 \text{(syst.)}$$

The results notably diverge from the Standard Model (SM) expectations, exceeding them by 2.0$\sigma$ for $R(D)$ and 2.7$\sigma$ for $R(D^*)$. Collectively, these findings present a discrepancy with the SM predictions at a level of 3.4$\sigma$.

Theoretical Implications

Semileptonic B decays involving the tau lepton ($\tau$) are particularly sensitive to contributions from potential new physics, such as charged Higgs bosons predicted by extensions of the SM like the type II Two-Higgs-Doublet Model (2HDM). The outcomes of this analysis demonstrate ratios that cannot be succinctly explained by the type II 2HDM, even within the optimistic model parameter space. This negation is grounded in the calculated values, which exclude the presence of a charged Higgs consistent with this particular extension.

Given the significant deviation of the experimental measurements from the SM, further theoretical evaluations could consider alternative models beyond the SM or explore additional contributions that might affect these decay processes.

Statistical and Systematic Considerations

The statistical significance of the signal was quantified at 6.8$\sigma$, marking this study as the first to secure such a substantial see above 5$\sigma$ for these decay processes. Systematic uncertainties include those due to background modeling, the calibration of the branching fractions of four B backgrounds, and the efficiency discrepancies between data and simulation samples, contributing to an overall reduced uncertainty in the final results.

Practical Implications and Future Research Directions

This study's results highlight a discrepancy in current models of particle physics and suggest potential paths for further exploration in the theoretical understanding of semileptonic B meson decays. Future research could focus on investigating other new physics contributions or on refining the existing SM predictions to resolve the discrepancy.

Moreover, further experimental efforts at current and upcoming high-luminosity colliders will be vital in confirming or refuting these findings, potentially uncovering new physical phenomena.

In conclusion, these findings not only affirm the BA$\mathbb{B}\text{AR}$ collaboration's capabilities in advancing particle physics knowledge but also open compelling questions regarding the completeness and comprehensiveness of the SM in light of new experimental evidence.