Andrea Fodor ; Andreas Warburton

2024
McGill University Montreal

Abstract: This thesis presents the first inclusive untagged analysis of $B \to X_u \ell \nu$ decays using data from the Belle II experiment. Within the Standard Model theory of particle physics, these are weak decays of the $B$ meson to a hadron containing an up quark, $X_u$, a charged lepton (either an electron or a muon), $\ell$, and the corresponding lepton neutrino, $\nu$. The decay rate is governed by the Cabbibo-Kobayashi-Maskawa quark mixing matrix element $V_{ub}$. The value of the $V_{ub}$ matrix element is one of the fundamental parameters of nature, a quantity that cannot be theoretically predicted and therefore must be measured. The $V_{ub}$ value can be extracted based on the measured $B \to X_u \ell \nu$ partial branching fraction. In this thesis, the measurement of the $B \to X_u \ell \nu$ branching fraction is done inclusively, whereby the outgoing hadron is not reconstructed. Only the charged lepton is selected. To avoid the dominant $B \to X_c \ell \nu$ background, in which $X_c$ is a hadron containing a charm quark, the signal yield is measured in the lepton momentum endpoint region, between 2.1 and 2.7~GeV. For the first time in the $B$-factory era of flavour physics studies, this measurement is uniquely performed using both the electron and muon final states, an approach not used before in previous $B$-factory inclusive measurements of these decay modes.\\ The analysis makes use of $(198.0 \pm 3.0) \times 10^6$ $B$ meson pairs recorded by the Belle II experiment. The Belle II experiment is a $B$-factory experiment located at the SuperKEKB electron-positron collider in Japan. It started collecting physics-quality collision data in 2019. \\ Using the recorded data and a Monte Carlo simulated data sample, the signal selection is developed. The signal yield is measured using a fit to the lepton momentum, and the systematic uncertainties are evaluated. The analysis procedure is validated by measuring the hadron-flavour agnostic $B \to X \ell \nu$ branching fraction. In this thesis, the sensitivities for the measurement of the $B \to X_u \ell \nu$ partial branching fraction and the subsequently extracted $V_{ub}$ value are presented. The expected experimental uncertainty for the partial branching fraction in the electron channel, $\Delta \mathcal{B}(B\to X_u e \nu)$, is $0.14\%_{\text{stat}} \pm 15.5\%_{\text{syst}}$, and in the muon channel, $\Delta \mathcal{B}(B\to X_u \mu \nu)$, is $0.16\%_{\text{stat}} \pm 12.0\%_{\text{syst}}$. The sensitivity in this early data-taking period is limited by systematic effects due to dominant particle identification uncertainties. The overall systematic uncertainty is expected to improve as the Belle II experiment collects more data and deeper understanding of the detector performance, motivated in part by this first study, is established.

Note: Presented on 25 05 2024
Note: PhD