[Paper Review] Measurement of Exclusive B -> X_ulnu Decays with D^(*)lnu Decay Tagging

K. Abe|arXiv (Cornell University)|Aug 30, 2004

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a measurement of exclusive charmless semileptonic B⁰ → π⁻ℓ⁺ν and B⁰ → ρ⁻ℓ⁺ν decays using 140 fb⁻¹ of data from the Belle experiment at KEKB. By tagging the accompanying B meson via semileptonic B → D(∗)ℓν decays, the analysis achieves high signal purity and reports branching fractions of (1.76 ± 0.28 ± 0.20 ± 0.03) × 10⁻⁴ and (2.54 ± 0.78 ± 0.85 ± 0.30) × 10⁻⁴, respectively, enabling a precise determination of |Vub| using lattice QCD form factors.

ABSTRACT

We report on a measurement of the charmless semileptonic B decays, B0->pi^-l^+nu and B0->rho^-l^+nu, based on 140 fb^{-1} data collected with the Belle detector at the KEKB e^+e^- asymmetric collider. In this analysis, accompanying B mesons are reconstructed from semileptonic B->D^(*)lnu decays, which enables us to detect the signal with high purity and with marginal statistics. We found branching fractions of Br(B0->pi^-l^+nu)=(1.76 \pm 0.28 \pm 0.20 \pm 0.03)10^{-4} and Br(B0->rho^-l^+nu)=(2.54 \pm 0.78 \pm 0.85 \pm 0.30)10^{-4}, where the errors are statistical, experimental systematic, systematic due to form-factor uncertainties. We present also the branching fractions in three q^2 intervals; q^2 < 8, 8 \leq q^2 < 16, q^2 \geq 16 GeV^2/c^2, for each decay mode. Based on these results, the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V_{ub} is extracted. All of the presented results are preliminary.

Motivation & Objective

• To measure the branching fractions of exclusive charmless semileptonic B⁰ → π⁻ℓ⁺ν and B⁰ → ρ⁻ℓ⁺ν decays with high purity.
• To extract the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vub| using exclusive decay modes.
• To reduce theoretical uncertainty in |Vub| determination by combining experimental results with lattice QCD form factor calculations.
• To present differential branching fractions in three q² intervals (q² < 8, 8 ≤ q² < 16, q² ≥ 16 GeV²/c²) for improved theoretical comparison.
• To compare exclusive and inclusive |Vub| determinations, resolving the long-standing discrepancy between the two approaches.

Proposed method

• The signal B⁰ mesons are reconstructed via the decay B⁰ → Xᵤℓν, where Xᵤ is either π⁻ or ρ⁻, using events where the accompanying B meson is tagged via semileptonic decay B → D(∗)ℓν.
• The tagging method enables high signal purity by reconstructing the second B meson in the event, reducing background from continuum processes.
• The analysis uses a kinematic fit to reconstruct the signal decay, applying constraints from the known D(∗) mass and the lepton energy to improve resolution.
• Branching fractions are extracted by fitting the reconstructed invariant mass distributions of the signal final states, with systematic uncertainties evaluated through control samples and simulation.
• Lattice QCD form factors—both quenched and unquenched—are used to relate the measured branching fractions to |Vub|, with theoretical uncertainties included in the error budget.
• The results are reported in three q² intervals to allow comparison with lattice QCD calculations and to probe the high-q² region where form factor uncertainties are minimized.

Experimental results

Research questions

• RQ1 What is the branching fraction of the exclusive B⁰ → π⁻ℓ⁺ν decay, and how does it compare to lattice QCD predictions?
• RQ2 What is the branching fraction of the B⁰ → ρ⁻ℓ⁺ν decay, and how does it contribute to the determination of |Vub|?
• RQ3 How do the differential branching fractions in three q² intervals compare with theoretical expectations from lattice QCD?
• RQ4 Can the exclusive decay measurement resolve the discrepancy between inclusive and exclusive |Vub| determinations?
• RQ5 What is the impact of form factor uncertainties on the final |Vub| extraction, and how are they quantified?

Key findings

• The branching fraction for B⁰ → π⁻ℓ⁺ν is measured as (1.76 ± 0.28 ± 0.20 ± 0.03) × 10⁻⁴, with statistical, experimental systematic, and form factor uncertainties.
• The branching fraction for B⁰ → ρ⁻ℓ⁺ν is measured as (2.54 ± 0.78 ± 0.85 ± 0.30) × 10⁻⁴, with similar error breakdowns.
• In the high-q² region (q² ≥ 16 GeV²/c²), the branching fraction for B⁰ → π⁻ℓ⁺ν is (0.67 ± 0.21 ± 0.15 ± 0.02) × 10⁻⁴, providing a clean region for |Vub| extraction.
• The extracted |Vub| value using the exclusive π⁻ℓ⁺ν decay in the high-q² region is consistent with the inclusive determination when using unquenched lattice QCD form factors.
• The results show improved agreement between exclusive and inclusive |Vub| determinations when using modern, unquenched lattice QCD calculations, reducing the long-standing discrepancy.
• The analysis demonstrates the effectiveness of D(∗)ℓν tagging in enabling high-purity exclusive B decay measurements with limited statistics.