Nadia Toutounji ; Kevin Varvell ; Bruce Yabsley

2024
University of Sydney Sydney

Abstract: Precision measurements of the magnitudes of the Cabibbo-Kobayashi-Maskawa (CKM)-matrix elements, which characterise the transitions between quarks of different flavours, are essential for testing the unitarity of the Unitarity Triangle (UT) described by the Standard Model. Of particular interest is the measurement of the magnitude of the matrix element $|V_{\mathrm{ub}}|$, which forms a significant uncertainty in global UT fits. $|V_{\mathrm{ub}}|$ can be measured via an exclusive approach from the differential decay rate of the semi-leptonic $B$-meson decay $B\to\pi\ell\nu_\ell$, where $\ell$ refers to a light lepton, either an electron $e$ or muon $\mu$. Using early data collected by the Belle II Experiment located in Tsukuba, Japan, an experiment based on the second-generation $B$-meson factory SuperKEKB which produces millions of pairs of $B$-mesons from electron-positron collisions, this study investigates the reconstruction of the signal $B\to\pi\ell\nu_\ell$ decay using a hadronic tagged approach. Using a state-of-the-art tagging algorithm developed for Belle II known as the Full Event Interpretation (FEI), $e^+e^- \to \Upsilon$(4S) $\to B\bar{B}$ events are reconstructed with the other, non-signal $B$-meson in the event explicitly reconstructed in one of a number of specified hadronic decay modes. The signal yields in a preliminary subset of Belle II data equivalent to an integrated luminosity of 362 fb$^{-1}$ are estimated via a binned maximum likelihood fitting procedure using predictions from simulation. The partial branching fractions for the decays $B^0\to\pi^-\ell^+\nu_\ell$ and $B^+\to\pi^0\ell^+\nu_\ell$ are extracted in bins of the square of the momentum transfer to the leptonic system, $q^2$, from which the total branching fractions are determined. Finally, measurements of $|V_{\mathrm{ub}}|$ are extracted using the distributions of the measured partial branching fractions alongside recent form factor constraints provided by lattice quantum chromodynamics (LQCD).

Note: Presented on 12 04 2024
Note: PhD