Search for $B^0 \rightarrow K^{*0}\tau^{\pm}\ell^{\mp}$ lepton flavour violating decays and tracking performance studies at the Belle and Belle II experiments

Sumitted to PubDB: 2025-10-16

Category: Phd Thesis

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Principal Authors Clotilde Lemettais
Date 2025-10-16
Belle II Number BELLE2-PTHESIS-2025-023
Abstract This thesis presents the search for $B^0 \rightarrow K^{*0} \tau^\pm \ell^\mp$ lepton flavour violating decays, where $\ell$ is either an electron or a muon. The analysis is performed using 365\invfb and 711\invfb datasets collected at the $\Upsilon(4S)$ resonance by the Belle II and Belle experiments, respectively. The study relies on an exclusive hadronic $B$-tagging technique, allowing the full reconstruction of the tagged $B$ meson. The signal is then searched for in the $\tau$ recoil mass system, corresponding to the system recoiling against the fully reconstructed tagged $B$ and the $K^{*0} \ell$ system. The background rejection process is performed in two steps with first a cut-based selection aiming at targeting specific and localised background sources, and then using a Boosted Decision Tree to reject the remaining background. The branching fractions $\mathcal{B}(B^0 \rightarrow K^{*0} \tau^\pm \ell^\mp)$ are extracted from a simultaneous maximum likelihood fit to the $\tau$ recoil mass distribution in Belle and Belle II data. No evidence for $B^0 \rightarrow K^{*0} \tau^\pm \ell^\mp$ decays is found, and upper limits on the branching fractions are set in the range of (2.9--6.4)$\times10^{-5}$ at 90\% confidence level using a CL$_\text{s}$ asymptotic method. In addition, two studies on Belle II tracking performance are presented. The first study is dedicated to the optimisation of the Deterministic Annealing Filter (DAF) algorithm used to improve the accuracy of track reconstruction. The second one focuses on the fake tracks rate at Belle II. Its evolution is measured with respect to the previous measurements, and the impact of exploiting the silicon vertex detector (SVD) timing information on the fake tracks rate is studied.

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