Measurement of inclusive (Formula presented) partial branching fractions and |V_ub| at Belle II

A sample of 365 fb⁻¹ of (Formula presented) data collected by the Belle II experiment is used to measure the partial branching fractions of charmless semileptonic B meson decays and determine the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element V_ub. Events containing a signal electron or muon l and a fully reconstructed hadronic B decay that constrains the signal kinematics are selected, while the rest of the event defines the hadronic system X_u associated with the signal. To discriminate the signal from the 50-times larger background originating from CKM-favored semileptonic B decays, a template fit is performed in both signal and control regions after applying an optimized selection. The partial branching fraction measured for lepton energies greater than 1 GeV in the signal B meson rest frame is (Formula presented). From this measurement, using the Gambino, Giordano, Ossola, Uraltsev theoretical framework, (Formula presented) is determined, where the uncertainties are experimental and theoretical, respectively. This value is consistent with the world average obtained from previous inclusive measurements. Different theoretical predictions and partial branching fractions measured in other phase-space regions, defined by additional selections on the X_u and leptonic system masses, are also used to determine jV_ubj. This allows for a comparison of the resulting values across theoretical frameworks and phase-space regions.