Thesis BELLE2-PTHESIS-2023-001

Search for τ → ℓϕ lepton flavour violating decays and fake tracks rate measurement with e+ e− → τ+ τ− events at the Belle II experiment

Léonard Polat ; Justine Serrano

CPPM Marseille

Abstract: We report the search of lepton flavour violating τ → ℓϕ 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 s = √10.58 GeV. We look for signal signatures that could result from e+ e− → τ+ τ− events, by reconstructing one tau going into a charged lepton and a meson ϕ → K+ K−, while the oppositely charged tau is left unconstrained to maximise the signal selection efficiency. Using Monte Carlo simulation, we define a background suppression strategy that combines cut-based selections and boosted decision trees, taking as inputs kinematic, topological and other event related variables. These selections are tuned in order to minimise 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 upper limits on τ → eϕ and τ → μϕ branching fractions of the order of 10^(−7). At equivalent luminosities, our result on the muon decay mode is comparable to the expected limit of the previous Belle experiment’s analysis and slightly better than the one from the BaBar experiment (−17%). In addition, we conduct a measurement of the rate of fake tracks in e+ e− → τ+ τ− events. To this end, we devise an original method applied on τ-pairs decaying into four tracks, with the possible presence of an additional fake one. This rate, measured in data and simulation, is found to be around 1% in both cases.

Note: Presented on 14 12 2022
Note: PhD

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 Record created 2023-01-19, last modified 2023-01-19

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