[Paper Review] Measurement of |V_cb| and the form-factor slope for Bbar -> D l^- nubar_l decays on the recoil of fully reconstructed B mesons

B. Aubert|arXiv (Cornell University)|Jul 31, 2008

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a precise measurement of the CKM matrix element |V_cb| and the form-factor slope ρ² in B̄ → D l⁻ ν̄_l decays using 417 fb⁻¹ of data from the BaBar experiment at the Upsilon(4S) resonance. By fully reconstructing the hadronic decay of the second B meson, the study determines G(1)|V_cb| = (43.0 ± 1.9 ± 1.4) × 10⁻³ and, using lattice QCD for G(1), extracts |V_cb| = (39.8 ± 1.8 ± 1.3 ± 0.9) × 10⁻³, with the final error including form factor uncertainty.

ABSTRACT

We present a measurement of the CKM matrix element |V_cb| and the form-factor slope rho^2 for Bbar -> D l^- nubar_l decays based on 417 fb-1 of data collected at the Upsilon(4S) resonance with the BaBar detector. The semileptonic decays are selected in BBar events in which the hadronic decay of the second B meson is fully reconstructed. From the measured differential decay rate of the signal decay we determine G(1) |V_cb|= (43.0 +/- 1.9 +/- 1.4) x 10^-3, rho^2 = 1.20 +/- 0.09 +/- 0.04, where G(1) is the hadronic form factor at the point of zero recoil. Using a lattice calculation for G(1) we extract |V_cb|= (39.8 +/- 1.8 +/- 1.3 +/- 0.9) x 10^-3, where the stated errors refer to the statistical, systematic, and form factor uncertainties. We also present a measurement of the exclusive branching fractions, BF(B^- -> D^0 l^- nubar_l) = (2.31 +/- 0.08 +/- 0.07)% and BF (B0bar -> D^+ l^- nubar_l)=(2.23 +/- 0.11 +/- 0.08)%.

Motivation & Objective

• To determine the CKM matrix element |V_cb| with improved precision using semileptonic B̄ → D l⁻ ν̄_l decays.
• To measure the form-factor slope ρ² in the kinematic region near zero recoil.
• To reduce uncertainties in |V_cb| by combining experimental data with lattice QCD calculations for the hadronic form factor G(1).
• To provide updated exclusive branching fraction measurements for B⁻ → D⁰ l⁻ ν̄_l and B̄⁰ → D⁺ l⁻ ν̄_l decays.

Proposed method

• The analysis uses 417 fb⁻¹ of data collected at the Upsilon(4S) resonance with the BaBar detector.
• Semileptonic B̄ → D l⁻ ν̄_l decays are selected in events where the hadronic decay of the second B meson is fully reconstructed.
• The differential decay rate is measured as a function of q², the invariant mass squared of the lepton-antilepton system.
• The form factor G(1) at zero recoil is extracted from the measured decay rate, and lattice QCD calculations are used to determine G(1) independently.
• The value of |V_cb| is extracted by combining the measured G(1)|V_cb| with the lattice QCD result for G(1).
• Exclusive branching fractions are measured by normalizing the signal yield to the total number of reconstructed B mesons.

Experimental results

Research questions

• RQ1 What is the precise value of the CKM matrix element |V_cb| in B̄ → D l⁻ ν̄_l decays, with reduced theoretical and experimental uncertainties?
• RQ2 What is the slope ρ² of the D-meson form factor near zero recoil, and how does it constrain theoretical models of semileptonic B decays?
• RQ3 How do the exclusive branching fractions for B⁻ → D⁰ l⁻ ν̄_l and B̄⁰ → D⁺ l⁻ ν̄_l compare with theoretical expectations?
• RQ4 To what extent can lattice QCD calculations reduce the uncertainty in |V_cb| when combined with high-precision experimental data?

Key findings

• The measured value of G(1)|V_cb| is (43.0 ± 1.9 ± 1.4) × 10⁻³, where G(1) is the hadronic form factor at zero recoil.
• The form-factor slope is determined to be ρ² = 1.20 ± 0.09 ± 0.04.
• Using a lattice QCD calculation for G(1), the extracted value of |V_cb| is (39.8 ± 1.8 ± 1.3 ± 0.9) × 10⁻³, with errors from statistics, systematics, and form factor uncertainty.
• The exclusive branching fraction for B⁻ → D⁰ l⁻ ν̄_l is measured as (2.31 ± 0.08 ± 0.07)%.
• The exclusive branching fraction for B̄⁰ → D⁺ l⁻ ν̄_l is measured as (2.23 ± 0.11 ± 0.08)%.
• The results represent a significant improvement in precision over previous measurements, particularly due to the use of fully reconstructed B mesons and lattice QCD input.