Search for CP violaton in τ decays at Belle II

Abstract

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 integrated luminosity 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 is performed using a similar method, with improved control over detector and background-induced asymmetries 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 are derived from control samples, and a dilution factor is applied to suppress contributions from background modes such as τ− → K−K0Sντ and τ− → π−K0 ¯K0ντ . The analysis is conducted separately for electron- and muon-tagged τ decays, totaling approximately 262 × 103 signal events, and statistically combined assuming uncorrelated systematic uncertainties. In parallel, this thesis contributes to a theoretical study evaluating the Standard Model assumption that the CP-violating term from τ− → π−K0 ¯K 0ντ decays vanishes. The K0S efficiency functions developed here are essential for estimating the A3 contribution, enabling comparison between theoretical predictions and Monte Carlo simulations. For the main measurement, the dominant uncertainty is statistical, followed by systematics from detection-induced asymmetry corrections. At the time of writing, the CP asymmetry result remains blinded pending internal Belle II review; however, the associated statistical and systematic uncertainties are presented. This work highlights Belle II’s sensitivity to CP violation in τ decays and lays the groundwork for more precise future measurements.