Search for CP violation in τ decays atBelle I

This thesis presents a search for direct Charge-Parity (CP) violation in the decay τ− →π−K0Sντ (≥ 0π0) using data collected by the Belle II experiment, corresponding to an integratedluminosity of 365 fb−1. The analysis is motivated by a previous BABAR measurement, which reported a 2.8σ deviation from the Standard Model prediction. A CP asymmetry measurement isperformed using a similar method, with improved control over detector and background-inducedasymmetries through dedicated corrections.The observed decay-rate asymmetry between τ− and τ+ decays is corrected for detection effects,background biases, and known K0/K¯ 0 nuclear interaction asymmetries. These corrections arederived from control samples, and a dilution factor is applied to suppress contributions frombackground modes such as τ− → K−K0Sντ and τ− → π−K0K¯ 0ντ .The analysis is conducted separately for electron- and muon-tagged τ decays, totaling approximately 262 × 103signal events, and statistically combined assuming uncorrelated systematicuncertainties.In parallel, this thesis contributes to a theoretical study evaluating the Standard Model assumption that the CP-violating term from τ− → π−K0K¯ 0ντ decays vanishes. The K0Sefficiencyfunctions developed here are essential for estimating the A3 contribution, enabling comparisonbetween theoretical predictions and Monte Carlo simulations.For the main measurement, the dominant uncertainty is statistical, followed by systematicsfrom detection-induced asymmetry corrections. At the time of writing, the CP asymmetryresult remains blinded pending internal Belle II review; however, the associated statistical andsystematic uncertainties are presented. This work highlights Belle II’s sensitivity to CP violationin τ decays and lays the groundwork for more precise future measurements.