Jo-Frederik Krohn

31 July 2018

Abstract: The Belle II experiment is the upgrade of the Belle experiment, performed at the SuperKEKB asymmetric electron-positron-collider, located in Tsukuba, Japan. With an design instantaneous luminosity of $8\cdot 10^{35}\rm{cm}^{-2}\rm{s}^{-1}$ a dataset of $50\ \rm{ab}^{-1}$ will be collected. The clean collision environment of the experiment and the large dataset yields the ability to perform high precision measurements of physics mediated by the weak force, such as semileptonic $B$-meson decays. Of special focus of this document are magnitudes of the CKM matrix elements $\rm{V_{\rm{ub}}}$ and $\rm{V_{\rm{cb}}}$ measured in $B\to D^{(\star)} l \nu$ and $B\to \pi l \nu$, where $l = e,\mu$, as well as the ratio of the branching fractions, $R_{D^{(\star)}} := \mathcal{B} (B\to D^{(\star)} e \nu) / \mathcal{B} (B\to D^{(\star)} \mu \nu) $ and $R_{\tau} := \mathcal{B} (B\to D^{(\star)} \tau\nu) / \mathcal{B} (B\to D^{(\star)} l \nu) $ , of lepton couplings in these decays. Some of these have shown persistent tension with the Standard Model and are therefore of primary interest towards a better understanding of the weak force carriers and potential new physics couplings.