A novel analysis of $B\to D\ell\nu_{\ell}$ and $B\to D^{*}\ell\nu_{\ell}$ decays at Belle II

Sumitted to PubDB: 2025-05-25

Category: Phd Thesis, Visibility: Public

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Authors Mirco Dorigo, Michele Mantovano
Date 2025-01-23
Belle II Number BELLE2-PTHESIS-2025-016
Abstract This particle physics thesis presents the first simultaneous analysis of the $B \to D \ell \nu_{\ell}$ and $B \to D^{*} \ell \nu_{\ell}$ decays at the Belle II experiment, where $\ell$ denotes either an electron or a muon. The analysis uses electron-positron collision data produced by the SuperKEKB collider at the $\Upsilon(4S)$ resonance and collected by Belle II between 2019 and 2022. The Belle II experiment is designed to reconstruct the decays of $B$ and $D$ mesons and $\tau$ leptons, to provide high-precision measurements that test the heavy-flavour sector of the Standard Model of particle physics. The $B \to D \ell \nu_{\ell}$ and $B \to D^{*} \ell \nu_{\ell}$ decays are reconstructed from about 387 million $B\overline{B}$ pairs. I analyse the dynamics of these decays using novel observables, defined for the first time in this thesis, that enable a number of key results. The most significant of these is a precise measurement of the weak coupling strength between beauty and charm quarks, $|V_{cb}|$, a fundamental Standard Model parameter of the quark weak interactions. Additional measurements include four signal decay branching fractions and, for the $B\to D^{*}\ell \nu_{\ell}$ decay, the lepton forward-backward asymmetry $A_{\rm FB}$, and the $D^{*}$ longitudinal polarisation $F_{L}^{D^{*}}$. The analysis is performed on simulated and control-data samples and will be applied to the signal sample upon approval from the Belle II Collaboration following internal review. At the time of this writing, the measurement values remain blinded. The anticipated precisions are competitive with those of world's best results. In addition, by assuming isospin symmetry, I obtain from the same analysis a new measurement of $f_{+-}/f_{00}$, the ratio of the branching fractions of $\Upsilon(4S)$ decays into charged and neutral $B$-meson pairs, which is also expected to be competitive with the best measurements of this quantity. Improving the precision on $f_{+-}/f_{00}$ enhances the accuracy of all measurements of $B$ branching fractions at Belle II.

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