Thesis BELLE2-MTHESIS-2021-069

Construction of Angular Observables Sensitive to New Physics in $\bar B\to D^*\tau^-\bar \nu_\tau$ Decays and Measurements of Differential Cross Sections of $\bar B\to D^*\ell^-\bar\nu_\ell$ Decays with Hadronic Tagging at Belle

Kilian Lieret ; Thomas Kuhr ; Martin Jung ; Gerhard Buchalla

2018
Ludwig Maximilian University of Munich Munich

Abstract: Recent measurements of $\bar B\to D^{(*)}\ell^-\bar\nu_\ell$ cross sections at Belle, BaBar, and LHCb challenge lepton universality and thus the Standard Model at a combined confidence level close to four standard deviations. New measurements of differential decay rates could contribute to the understanding of these anomalies. The differential cross section of the decay $\bar{B} \longrightarrow D^{*}(\rightarrow D \pi) l^{-} \bar{\nu}_{l}$ is parametrized according to different dependencies on the three decay angles and the coupling constants of potential new physics contributions. Observables using binned measurements of the differential cross section are characterized and explicitly constructed. Based on an estimate for the obtainable sensitivity, optimal binnings for such measurements are discussed. The discriminatory power of the thus constructed observables is discussed based on a basis of dimension six operators with renormalizable couplings contributing to $\bar{B} \longrightarrow D^{*} l^{-} \bar{\nu}_{l}$. Furthermore, continuing work on an analysis of the $\bar{B} \longrightarrow D^{*}(\rightarrow D \pi) \ell^{-} \bar{\nu}_{\ell}$ decay channel for $\ell=e, \mu$ using data from the Belle detector at KEKB is presented. The events are selected from 772 million $e^{+} e^{-} \rightarrow \Upsilon(4 S) \rightarrow B \bar{B}$ events, where one $B$ meson is fully reconstructed in hadronic modes. Unfolded differential decay rates in four kinematic variables are presented separately for $\ell=e, \ell=\mu$ and a combined fit, allowing for precise calculations of $V_{c b}$ and $B \longrightarrow D^{*}$ form factors. The new lepton flavor specific results are also expected to impact the discussion about potential light lepton flavor universality violations prompted by measurements of $B \longrightarrow K^{(*)} \ell \ell$ decays.

Note: Presented on 16 09 2018
Note: MSc

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