Study of 𝑩 → 𝝅l𝝂 and 𝑩 → 𝝆l𝝂 decays and extraction of |𝑽𝒖𝒃| at Belle II

Sumitted to PubDB: 2024-11-27

Category: Phd Thesis, Visibility: Public

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Authors Florian Bernlochner, Jochen Dingfelder, Svenja Granderath, Peter Lewis
Date Jan. 1, 2024
Belle II Number BELLE2-PTHESIS-2024-022
Abstract This thesis presents a measurement of the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |Vub| from a simultaneous study of the semileptonic decays B0 → π−l+νl and B+ → ρ0l+νl, where l = e,μ. The analysis utilizes a data sample corresponding to 364 fb−1 of integrated luminosity, collected by the Belle II detector at the SuperKEKB electron-positron accelerator from 2019 to 2022. The SuperKEKB accelerator operates at a center-of-mass energy corresponding to the mass of the Υ(4S) resonance. This results in nearly all produced Υ(4S) mesons decaying into two B mesons, which then decay into lighter particles. An untagged measurement approach is employed in this thesis, where both decay modes are reconstructed without identifying the partner B mesons. After initial signal reconstruction and selection, machine learning methods, specifically boosted decision trees, are trained and applied to suppress background contributions. The differential branching fractions of B0 → π−l+νl and B+ → ρ0l+νl are measured simultaneously as functions of q2 (momentum transfer squared). This is achieved by introducing a novel fit method, in which yields are inherently corrected for finite detector resolution. The total branching fractions are determined to be B(B0 → π−l+νl) = (1.516 ± 0.042(stat) ± 0.059(syst)) × 10−4, B(B+ → ρ0l+νl) = (1.625 ± 0.079(stat) ± 0.180(syst)) × 10−4. Fitting the measured partial branching fractions of B0 → π−l+νl with lattice QCD constraints, and including additional constraints from light-cone sum rules (LCSR), results in |Vub| = (3.93 ± 0.09(stat) ± 0.13(syst) ± 0.19(theo)) × 10−3, |Vub| = (3.73 ± 0.07(stat) ± 0.07(syst) ± 0.16(theo)) × 10−3, respectively. For B+ → ρ0l+νl decays, applying LCSR constraints yields |Vub| = (3.19 ± 0.12(stat) ± 0.17(syst) ± 0.26(theo)) × 10−3. While the results are limited by theoretical uncertainties, the leading systematic uncertainties arise from the sizes of the available off-resonance and simulated samples.
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