Determination of |Vcb| using B → Dlνl decays in Belle II data

Sumitted to PubDB: 2025-05-02

Category: Phd Thesis

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Principal Authors Philipp Horak
Non-Belle II authors Philipp Horak
Date 2024-08-07
Belle II Number BELLE2-PTHESIS-2025-010
Abstract This thesis addresses the long-standing tension between exclusive and inclusive determi- nations of the CKM matrix element |Vcb| by focusing on the exclusive approach using B+ → D ̄0l+νl and B0 → D−l+νl decays. Leveraging data from the Belle II experiment, which began operations in 2019, a substantial data sample of B meson decays is analyzed. The thesis presents a preliminary result based on data collected between 2019 and 2021, and an updated measurement using a data sample approximately twice that size. A binned template fit is employed to extract the signal yields for the different modes in bins of the hadronic recoil w. The signal yields are unfolded and converted into differential decay rates. The theoretical form factor describes the shape of the differential decay rates as a function of w. A fit of the form factor to decay rates measured in experiment, with data from theory as additional constraining factor, allows measuring |Vcb|. Fitting the form factor in the parameterization by Boyd, Grinstein and Lebed to a spectrum of differential decay rates obtained from reconstructing B → Dlνl decays in a data sample corresponding to 189 fb−1 yields |Vcb| = (38.28 ± 1.16) × 10−3, with the error comprising uncertainties from statistical and systematic sources, as well as contributions from theoretical inputs. The result lies within one standard deviation of the world average across all exclusive determinations |Vcb| = (39.10 ± 0.50) × 10−3. The measurement is sensitive to |Vcb| at a total relative uncertainty of ∼3%, comparable to the Belle mea- surement of B → Dlνl decays, resulting in a total uncertainty of ∼2.8% [4] using a data sample approximately four times larger. The sensitivity estimate for the updated measurement projects a relative uncertainty reduction to approximately 2.1%, corresponding to world-leading precision for measuring |Vcb| with the Dlν final state. This improvement is attributed to, among other factors, an increased data sample, enhanced signal extraction, refined unfolding methods, and improved form factor fitting. In addition, light-lepton flavour universality can be measured by comparing branching ratios with electrons in the final state to those with muons, with an estimated total sensitivity on Re/μ of 2%.

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