NEPAL

We report the search of lepton flavour violating \( \tau \to \ell \phi\)decays at the Belle II experiment, based on an integrated luminosity Ldt = 189.88 fb−1 of data collected from 2019 to 2021, at the Υ(4S) meson resonance energy of 10.58 GeV. We look for signatures that could result from \( e^+e^- \to \tau^+ \tau^-\) events, by reconstructing one tau going into a charged lepton and a meson \(\phi \to K^+ K^- \), while the oppositely charged tau is left unconstrained to maximise the signal selection efficiency. Using MonteCarlo siulation, we defined a background suppression strategy that combines cut-based selection and boosted decision trees, taking as input kinematics, topological and other event variables. These selections are tunes in order to mininise as much as possible the expected upper limits on the branching fractions. Although the full data is not yet unblinded, we perform an estimation of the expected limis on \( \tau \to e \phi \) and \( \tau \to \mu \phi \) branching fractions of the order of \(10^{-7}\). At equivalent luminosites, our result on the muon decay mode is comparable to the expected limit of the previous Belle experiment analysis (+4%) and slightly better than the one from the Babar experiement (-13%). In addition, we conduct a measurement of the rate of fake tracks in \( e^+e^- \to \tau^+ \tau^-\) events. To this end, we devise an original method applied on tau pairs decaying into four tracks, with the possible presence of additional fake one. This rate, measure in data and simulation, is found to be around 1% in both cases.

This thesis presents the search for lepton flavour violating \( \tau \to \mu \mu \mu\) decays using \( e^+ e^- \to \tau^+\tau^- \) events at the Belle II experiment. The srtategy is based on an 'untagged' reconstruction of one tau going into three muons, while the opposite charged tau is left unconstrained to maximize the signal efficiency. A three-step strategy is adopted to reject background based on muon identification variables, cut-based selections and bossted decision trees, which take kinematic, topological, and other event-related variables as inputs. The background rejections is optimized using Monte Carlo simulated samples to minimize the upnzi figure-of-merit. After determining the number of expected events in data and the systematic uncertiantes, the expected limit on the \( \tau \to \mu \mu \mu\) branching fraction is expected to be 1.6 \( \times 10^{-8}\) at 90% confidence level using 424 \(fb^{-1} \). The previous Belle limit of 2.1 \( \times 10^{-8}\) is thus expected to be improved with a data sample twice smaller. In addition, the thesis also includes the measurement of the vertex detector cluster spatial resolution using overlapping sensors.

This thesis reports the search for lepton flavour violating decays \( \tau \to e \ell \ell\), where \( \ell=e,\mu \),using the Run 1 dataset of the Belle II experiment, corresponding to an integrated luminosity of 424 fb-1. The analysis is based on an untagged reconstruction of the signal candidates in events, where only the signal is reconstructed while the other is left unconstrained. The background suppression is performed on data sidebands due to the important contribution of QED processes (\( e^+e^- \to \ell^+\ell^-\ell^{+'}\ell^{-'} \) ) that are mis-modeled in the simulations. The analysis relies on a combination of selection criteria and a BDT classifier to achieve an almost background-free scenario, while maintaining a signal reconstruction efficiency in the range [15-24]%. The branching fraction is extracted with an unbinned maximum likelihood fit to the distribution, and the upper limits are computed using the method. The systematic uncertainties affecting the signal efficiency and the background modelling are evaluated. According to the expected limits evaluated using data sidebands, the analysis improves the sensitivity in three of the modes compared to the previous Belle's results obtained with 762 fb-1 , one mode is comparable to Belle's results and one mode is less sensitive. In addition, three studies of the fake tracks rate are included. The first one implements a track time variable based on measurement from the silicon vertex detector (SVD), the second one uses a classifier to reject detector hits based on the time information in degraded data acquisition conditions and the third one studies the fake tracks rate in the proposed upgrade of the vertex detector (VTX).