[Paper Review] Measurement of $\mathcal{R}(D)$ and $\mathcal{R}(D^{\ast})$ with a semileptonic tagging method

The Belle Collaboration, A. Abdesselam+463 more|arXiv (Cornell University)|Apr 18, 2019

Particle physics theoretical and experimental studiesPhysics and Astronomy66 citations

TL;DR

The Belle collaboration measures the ratios R(D) and R(D*) using a semileptonic tagging approach with a large Belle data sample, reporting values consistent with the Standard Model within uncertainties.

ABSTRACT

We report a measurement of the ratios of branching fractions $\mathcal{R}(D) = {\cal B}(\bar{B} o D τ^- \barν_τ)/{\cal B}(\bar{B} o D \ell^- \barν_{\ell})$ and $\mathcal{R}(D^{\ast}) = {\cal B}(\bar{B} o D^* τ^- \barν_τ)/{\cal B}(\bar{B} o D^* \ell^- \barν_{\ell})$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772 imes10^6$ $B\bar{B}$ events recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The analysis utilizes a method where the tag-side $B$ meson is reconstructed in a semileptonic decay mode, and the signal-side $τ$ is reconstructed in a purely leptonic decay. The measured values are $\mathcal{R}(D)= 0.307 \pm 0.037 \pm 0.016$ and $\mathcal{R}(D^{\ast})= 0.283 \pm 0.018 \pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within $0.2$ and $1.1$ standard deviations, respectively, while their combination agrees with the Standard Model predictions within $1.2$ standard deviations.

Motivation & Objective

• Motivate precision tests of b→cτντ decays as probes of new physics.
• Measure the ratios R(D) and R(D*) to reduce common systematics.
• Introduce and apply a semileptonic tagging method for these observables.
• Improve the R(D*) measurement by combining B0 and B+ decays with enhanced tagging.
• Provide results that test consistency with Standard Model predictions.

Proposed method

• Reconstruct tag-side B meson in a semileptonic decay mode.
• Reconstruct signal-side τ in a purely leptonic decay channel.
• Use a data sample of 772×10^6 B-B̄ events collected with the Belle detector at KEKB.
• Compare signal and normalization modes to form R(D) and R(D*) ratios.
• Extract results with statistical and systematic uncertainties separated.

Experimental results

Research questions

• RQ1 What are the measured values of R(D) and R(D*) using semileptonic tagging?
• RQ2 Do the measurements agree with Standard Model predictions within reported uncertainties?
• RQ3 How does the semileptonic tagging approach impact the precision compared to hadronic tagging?
• RQ4 What is the combined assessment of R(D) and R(D*) versus SM expectations?

Key findings

• R(D) = 0.307 ± 0.037 (stat) ± 0.016 (syst).
• R(D*) = 0.283 ± 0.018 (stat) ± 0.014 (syst).
• Both results are in agreement with the Standard Model within 0.2σ and 1.1σ, respectively.
• The combination of R(D) and R(D*) is in agreement with the SM within 1.2σ.