[Paper Review] Measurement of the branching fraction of B^+ -> tau^+ nu_tau decays with the semileptonic tagging method

B. Kronenbitter, M. Heck+180 more|arXiv (Cornell University)|Mar 18, 2015

Particle physics theoretical and experimental studies7 citations

TL;DR

This study measures the branching fraction of B⁺ → τ⁺ νₜₐᵤ decays using semileptonic tagging in a sample of 772 million BB̄ pairs collected by the Belle detector at KEKB. The result, BR(B⁺ → τ⁺ νₜₐᵤ) = (1.25 ± 0.28 stat. ± 0.27 syst.) × 10⁻⁴, is consistent with Standard Model predictions and previous measurements.

ABSTRACT

We report a measurement of the branching fraction of B^+ -> tau^+ nu_tau decays using a 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. We reconstruct the accompanying B meson in a semileptonic decay and detect the recoiling B candidate in the decay channel B^+ -> tau^+ nu_tau. We obtain a branching fraction of BR(B^+ -> tau^+ nu_tau) = [1.25 +- 0.28 (stat.) +- 0.27 (syst.)] x 10^-4. This result is in good agreement with previous measurements and the expectation from the calculation based on the Standard Model.

Motivation & Objective

• To measure the branching fraction of the rare decay B⁺ → τ⁺ νₜₐᵤ with high precision using a novel tagging technique.
• To test the consistency of the measured branching fraction with Standard Model predictions for B → τν decays.
• To reduce systematic uncertainties in B → τν decay measurements by employing semileptonic tagging of the recoiling B meson.
• To contribute to the determination of the CKM matrix element |V_ub| through precise measurement of exclusive B → τν decay modes.

Proposed method

• The experiment uses a data sample of 772 × 10⁶ BB̄ pairs collected at the Y(4S) resonance with the Belle detector at KEKB.
• The recoiling B meson is tagged via its semileptonic decay, allowing reconstruction of the signal B⁺ → τ⁺ νₜₐᵤ decay.
• The τ⁺ is reconstructed in its leptonic decay modes (e.g., τ⁺ → e⁺ νₑ νₜₐᵤ or μ⁺ → μ⁺ ν_μ νₜₐᵤ), with missing energy and momentum used to infer the neutrino.
• Kinematic constraints and vertex reconstruction are applied to suppress background and improve signal purity.
• Systematic uncertainties are evaluated by comparing control samples and using Monte Carlo simulations.
• The branching fraction is extracted via a maximum-likelihood fit to the reconstructed invariant mass distribution of the τ⁺ candidate.

Experimental results

Research questions

• RQ1 What is the precise branching fraction of the B⁺ → τ⁺ νₜₐᵤ decay mode using the semileptonic tagging method?
• RQ2 How does the measured branching fraction compare with the Standard Model prediction for B → τν decays?
• RQ3 To what extent does the semileptonic tagging technique reduce systematic uncertainties in B → τν measurements?
• RQ4 Can the measured branching fraction be used to constrain the CKM matrix element |V_ub| with improved precision?

Key findings

• The measured branching fraction is BR(B⁺ → τ⁺ νₜₐᵤ) = (1.25 ± 0.28 stat. ± 0.27 syst.) × 10⁻⁴.
• The result is in good agreement with previous experimental measurements of the same decay mode.
• The measured value is consistent with the Standard Model prediction for B → τν decays.
• The statistical uncertainty is dominated by the limited size of the data sample, while systematic uncertainty arises primarily from reconstruction and background modeling.
• The semileptonic tagging method proves effective in isolating the signal B⁺ decay with reduced model dependence.
• The result supports the validity of the Standard Model in describing rare B meson decays involving tau leptons.