Thesis BELLE2-PTHESIS-2024-001

Search for $\tau^-\to\mu^-\mu^+\mu^-$ lepton flavour violating decays and measurement of the SVD cluster position resolution at Belle II experiment

Robin Leboucher ; Justine Serrano

CPPM Marseille

Abstract: Keywords: Particle physics; Search for New Physics; Lepton Flavour Violation; Tau lepton decays; Spatial resolution; Vertex detector Lepton flavour violating decays are considered one of the most effective ways to search for physics beyond the standard model, as they are not allowed in the Standard Model of particle physics. A number of new physics models predict that the branching fractions of \tautomumumu are just below the current experimental limits. The \belletwo experiment, which operates at the KEK laboratory in Japan, has already collected a luminosity of 424\invfb between 2019 and 2022 at the \FourS \(\sqrt{s}=10.58\gev\) and \FiveS mesons resonance energies. In addition, \belletwo provides an ideal environment to study tau decays due to its clean environment and high \taupair cross-section. This thesis presents a search for \tautomumumu decays in \(\epem\to\tautau\) events. The strategy is based on an "untagged" reconstruction of one tau going into three muons, while the oppositely charged tau is left unconstrained to maximize the signal selection efficiency. A three-step strategy is adopted to reject background based on muon identification variables, cut-based selections, and boosted decision trees, which take kinematic, topological, and other event-related variables as inputs. The background rejection is optimized using Monte-Carlo simulated samples to minimize Punzi's figure of merit. After determining the expected number of data and the systematic uncertainty, the expected upper limit on \tautomumumu branching fractions is estimated to be \(1.56\times10^{-8}\) at 90\% confidence level using 424\invfb. Thus, the limit of \belle that reached \(2.1\times10^{-8}\) is expected to be improved with half of the luminosity. Moreover, the thesis also includes a measurement of the vertex detector spatial resolution using overlapping sensors. The \(\epem\to\mun\mup\) data event is selected to keep only the case where a particle has left two hits in the same detector layer. The method estimates the spatial resolution to be approximately between \(15\mum\) and \(32\mum\), depending on the layer and the sensor side.

Note: Presented on 31 10 2023
Note: PhD

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 Record created 2024-01-30, last modified 2024-02-05

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