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Topic

Study of Dalitz Decays from pi^{0} in e^{+}e^{−}→tau^{−}tau^{+} Events Recorded by the Belle II Detector as a Control Sample for Dark Sector Search Validation

Department of Physics and Astronomy

Date & location

• Tuesday, April 22, 2025
• 2:00 P.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Michael Roney, Department of Physics and Astronomy, University of Victoria (Supervisor)
• Dr. Robert Kowalewski, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Dr. Steven Robertson, Department of Physics, University of Alberta

Chair of Oral Examination

• Dr. Dennis Hore, Department of Chemistry, UVic

Abstract

This thesis presents a validation study of Monte Carlo (MC) simulations in the Belle II experiment by comparing simulated data with real experimental data along with the measurement of the Dalitz decay branching fraction (BF). The analysis focuses on the Dalitz decay of the neutral pion, 𝜋⁰→𝛾𝑒⁺𝑒⁻, which serves as a control sample for searches for dark photons in event 𝑒⁺𝑒⁻→𝛾[𝐴′→𝑒⁺𝑒⁻]. The neutral pions are from tau pair events where 𝜏⁺→𝜈̅_𝜏𝜋⁺[𝜋⁰→𝛾𝑒⁺𝑒⁻]. By evaluating efficiency and purity, we ensure the robustness of event selection, background suppression, and modeling of key kinematic distributions. After applying selection criteria, the computed MC purity is 99.27% with an efficiency of 0.0122, with the final data sample containing 572 events. The Dalitz decay BF is measured to 0.01076 ± 0.00045(stat.), in good agreement with the Particle Data Group (PDG) value of 0.01174 ± 0.00035, with a discrepancy of 1.7𝜎. This study is limited to evaluation of statistical uncertainties on the BF with a discussion on systematic studies as a follow up. These findings provide essential input for improving data-MC agreement in future dark photon searches and improving measurement of the Dalitz decay BF.