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

A measurement of partial branching fractions of charmless semileptonic beauty meson decaysusing an inclusively reconstructed hadronic system is presented. These decays are denoted asB → Xuℓν, where Xu represents a hadronic system containing a u quark produced in the un-derlying quark-level b → u transition. The measurement uses the full Run 1 Belle II samplecorresponding to 365 fb−1 of e+e− → Υ(4S) → B ¯B data. One of the two B mesons is recon-structed in a hadronic decay channel to constrain the signal kinematics. Events containing asignal electron or muon ℓ are selected, while the rest of the event defines the hadronic systemXu associated with the signal. To discriminate the signal from the 50-times larger backgroundoriginating from Cabibbo-favoured semileptonic B → Xcℓν decays, a template fit is performedin both signal and control regions after applying an optimised selection. The control regionsare used to constrain the kinematic properties of B → Xcℓν decays based on data and thereforereduce the dependence of the measurement on their modelling. The partial branching fractionmeasured 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 thebeauty-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 goodagreement with the world average of |Vub| = (4.06±0.16)×10−3. Different theoretical predictionsand partial branching fractions measured in other phase-space regions, defined by additionalselections on the Xu and leptonic system masses, are also used to determine |Vub|.