Experimental study of inclusive charmless semileptonic beauty meson decays at Belle II

Sumitted to PubDB: 2025-09-18

Category: Phd Thesis

Tags: Run 1

Principal Authors Tommy Martinov
Date 2025-09-11
Belle II Number BELLE2-PTHESIS-2025-020
Abstract A measurement of partial branching fractions of charmless semileptonic beauty meson decays using an inclusively reconstructed hadronic system is presented. These decays are denoted as B → Xuℓν, where Xu represents a hadronic system containing a u quark produced in the underlying quark-level b → u transition. The measurement uses the full Run 1 Belle II sample corresponding to 365 fb−1 of e+e− → Υ(4S) → BB data. One of the two B mesons is reconstructed in a hadronic decay channel to constrain the signal kinematics. Events containing a signal electron or muon ℓ are selected, while the rest of the event defines the hadronic system Xu associated with the signal. To discriminate the signal from the 50-times larger background originating from Cabibbo-favoured semileptonic B → Xcℓν decays, a template fit is performed in both signal and control regions after applying an optimised selection. The control regions are used to constrain the kinematic properties of B → Xcℓν decays based on data and therefore reduce the dependence of the measurement on their modelling. The partial branching fraction measured for lepton energies greater than 1 GeV in the signal B meson rest frame is ∆B(B → Xuℓν) = (1.54 ± 0.08 ± 0.12) × 10−3, where the uncertainties are statistical and systematic, respectively. From this measurement, using the Gambino, Giordano, Ossola and Uraltsev theoretical framework, the strength of the beauty-to-up quark coupling is determined to be |Vub| = (4.01 ± 0.19 +0.07 −0.08) × 10−3, where the uncertainties are experimental and theoretical, respectively. This value is in good agreement with the world average of |Vub| = (4.06±0.16)×10−3. Different theoretical predictions and partial branching fractions measured in other phase-space regions, defined by additional selections on the Xu and leptonic system masses, are also used to determine |Vub|.

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