Semileptonic and leptonic B decay results from early Belle II data

Abstract

The Belle II experiment at the SuperKEKB energy-asymmetric $e^+e^-$ collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is $8\times10^{35}\, \mathrm{cm}^{-2}\mathrm{s}^{-1}$ and the Belle II experiment aims to record $50\, \mathrm{ab}^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, SuperKEKB has completed a commissioning run, achieved a peak luminosity of $5.5 \times 10^{33}\, \mathrm{cm}^{-2}\mathrm{s}^{-1}$, and Belle II recorded a data sample of about $0.5\, \mathrm{fb}^{-1}$. In this presentation we show first results from studying missing energy signatures, such as leptonic and semileptonic B meson decays based on this early Belle II data. We report first studies on re-measuring important standard candle processes, such as the abundant inclusive $B\rightarrow X l \nu$ and $B\to D^*\ell\nu$ decays, and evaluate the performance of machine learning-based tagging algorithms. Furthermore, we also present an overview of the semileptonic B decays that will be measured in the upcoming years at Belle II and discuss prospects for important B-anomalies like R$(D)$ and R$(D^*)$, as well as other tests of lepton flavor universality.