Measurement of the branching ratio of $\bar{B} \to D^{(\ast)} τ^- \barν_τ$ relative to $\bar{B} \to D^{(\ast)} \ell^- \barν_\ell$ decays with hadronic tagging at Belle

Abstract: We report a measurement of the branching fraction ratios R(D()) of Bbar -> D() tau- nubar_tau relative to Bbar -> D()* l- nubar_l (where l = e or mu) using the full Belle data sample of 772 x 10^6 BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The measured values are R(D)= 0.375 +- 0.064(stat.) +- 0.026(syst.) and R(D*) = 0.293 +- 0.038(stat.) +- 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic tau decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.

• The paper presents precise measurements of the branching fractions R(D) and R(D*) in B meson decays using Belle’s complete dataset of 772 million B pairs.
• It employs hadronic tagging to differentiate semileptonic decays involving tau leptons from those with lighter leptons, enhancing measurement accuracy.
• The results, consistent with Standard Model predictions, refine constraints on new physics scenarios and limit potential contributions from charged Higgs effects.

Measurement of the Branching Ratio of $B$ Relative to $B$ Decays with Hadronic Tagging at Belle

The investigation presented in this paper offers a meticulous measurement of the branching fraction ratios $R(D)$ and $R(D^*)$ of $B$ meson decays relative to anti-$B$ meson decays. This analysis was conducted using the complete dataset from the Belle experiment, encompassing $772 \times 10^6 B\bar{B}$ pairs, collected at the $\Upsilon(4S)$ resonance by the Belle detector at KEKB, an asymmetric-energy $e^+ e^-$ collider.

The study focuses on semileptonic $B$ decays where light leptons ($e$ or $\mu$) are replaced by the higher-mass $\tau$ lepton. The interest lies in the sensitivity of these decays to potential new physics (NP) effects, such as the involvement of charged Higgs bosons, due to enhanced coupling to $\tau$ leptons predicted by some theoretical models. Evolution in these decay patterns could indicate deviations from the Standard Model (SM) predictions and possibly hint at new physics contributions.

Methodology and Measurements

1. Data and Detector Configuration: The analysis utilizes the full Belle data set, which comprises $772 \times 10^6 B\bar{B}$ pairs. The Belle detector operates with a magnetic spectrometer including a silicon vertex detector and central drift chamber, augmented with various particle identification devices.
2. Branching Fraction Ratios: The ratio $R(D)$ is defined as $$\mathcal{B}(B \to D \tau \nu_\tau) / \mathcal{B}(B \to D \ell \nu_\ell)$$, and similarly for $R(D^*)$, where $\ell$ corresponds to $e$ or $\mu$. The measurement proceeds through hadronic tagging of the companion $B$ meson and detection of purely leptonic $\tau$ decays.
3. Results: The measured values are $$R(D)=0.375 \pm 0.064\mathrm{(stat.)}\pm 0.026\mathrm{(syst.)}$$ and $$R(D^*)=0.293 \pm 0.038\mathrm{(stat.)}\pm 0.015\mathrm{(syst.)}$$. These results adhere closely to previous experimental data and demonstrate no significant deviation from the SM predictions, thus providing no current evidence for NP within the explored parameter space.

Implications and Future Prospects

The ratios $R(D)$ and $R(D^*)$ serve as crucial observables for testing the feasibility of various NP scenarios. This paper's results add significant data to the prevailing observations, demonstrating measurements consonant with SM estimates. Although this particular experiment does not capture any divergences suggestive of NP, it refines the precision in the determination of these important ratios.

Further exploration of semileptonic $B$ decays, especially those including $\tau$ leptons, continues to be a pivotal pursuit. Not only could it critically examine the anomalies observed by other experiments like BaBar and LHCb, but it also underpins larger goals of uncovering physics beyond the SM. Continuing advances in experimental precision and theoretical modeling, alongside the development of new experiments such as Belle II, can potentially capture subtler NP effects or validate SM accuracy in greater detail.

Conclusion

The effort expounded in this manuscript provides valuable contributions to the study of $B$ meson decay processes, particularly given the extensive dataset leveraged for analysis. As the field advances towards higher precision and broader datasets, particularly with collaborative global experiments, the nuanced understanding of these decay modes will invariably enhance our grasp of fundamental particle interactions.

The paper punctuates the necessity for continued investigation into $B$ decays, urging meticulously refined experimental techniques and cross-verification with theoretical predictions, all pivotal in the relentless pursuit of potential new physics insights.