τ -Lepton Lifetime Measurement and Working Point Optimization for the Pixel Vertex Detector at Belle II

Sumitted to PubDB: 2025-07-20

Category: Phd Thesis

Tags: PXD

Principal Authors Anselm Baur
Date 2025-05-14
Belle II Number BELLE2-PTHESIS-2025-018
Abstract In the Standard Model of Particle Physics, all leptons have the same coupling strength to the weak interaction. Comparing the theoretical model with experimental observations reveals a slight deviation in the electron branching fraction of τ-lepton decays. However, the precision of the measurement is limited by the precision of the measured τ-lepton lifetime. This work presents a novel measurement of the τ-lepton lifetime using a template fitting approach applied to 3×1-prong τ⁺τ⁻ decays at Belle II. The decay length is determined in the xy-plane from the precisely known SuperKEKB interaction point to the 3-prong τ decay vertex. A robust software framework was developed to unify data processing, validation, and systematic studies, ensuring reproducibility and flexibility for future improvements. The vertex reconstruction achieves a resolution of 31.43 ± 0.01 µm, leveraging the precision of the pixel vertex detector. Templates corresponding to different lifetime hypotheses are generated using a re-weighting method. The reliability of this approach is confirmed through comparisons with templates produced from shifted generator lifetimes and pseudo-data fits. Systematic uncertainties are incorporated into the likelihood fit model via nuisance parameters. To address dominant modeling uncertainties, a dedicated two-dimensional re-weighting strategy was developed, resulting in an expected total precision of 0.2 fs, including a statistical uncertainty of 0.08 fs and a systematic uncertainty of 0.18 fs. With this, the expected precision of the analysis exceeds the current world average by more than a factor of two. In 2023, the Belle II experiment upgraded its pixel vertex detector by replacing the previous single-layer configuration with a new two-layer detector, based on the same sensor design. Optimized sensor working points are crucial for the success of future analyses relying on precise vertex information with the new two-layer pixel vertex detector. To maximize hit efficiency, dedicated optimization studies were conducted. During pre-commissioning, detailed multi-parameter source scans were performed to evaluate and tune the sensor settings across half of the pixel vertex detector modules. These scans identified stable operation points that significantly improved hit efficiency, with gains of up to 14 %, and mitigated effects such as cluster anomalies in under-depleted modules. Simplified high-voltage scans with all pixel vertex detector modules were subsequently performed, resulting in suitable operating parameters that also serve as starting values for future in-situ calibration during beam collisions. Early data following the resumption of beam operations in 2024 revealed individual module improvements of up to 8 % from refined operation parameters.

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