SEARCH FOR B + → K + τ + τ − AT THE BELLE EXPERIMENT AND PROSPECTS AT BELLE II

Sumitted to PubDB: 2025-11-06

Category: Phd Thesis

Tags: KLM Belle

Principal Authors Christopher Ketter
Date 2024-12-17
Belle II Number BELLE2-PTHESIS-2025-027
Abstract The branching fraction for the electroweak penguin decay B → Kτ τ has not been measured. Motivated by speculative physics scenarios that predict a large enhancement in the branching fraction of B → Kτ τ compared to the Standard Model, and motivated further by recent success of the inclusive tagging technique, we develop an inclusive-hadronic-tagging technique for measuring the decay B + → K + τ + τ − . We use simulated Belle data equivalent in size to the 711 fb −1 recorded by the Belle experiment. The goal is to show that we can improve sensitivity over existing hadronic- tagging measurements of this decay. Based on simulation, we project an upper limit for B(B + → K + τ + τ − ) < 1.14 × 10 −4 at 90 % confidence level, a factor of about 20 better than the world’s best constraint. The second part of this dissertation addresses a critical weakness in τ -lepton reconstruction, which is muon identification for τ decays to muon final states. As Belle II aims to record 50 ab −1 at the Υ(4S) resonance, it is of utmost importance that such calibration work is done now in order to enable future searches for this and other rare B-meson decays. Some of this poor muon identification can be attributed to lack of calibration and to unused features of the readout electronics for the Belle II K-Long and Muon (KLM) detector subsystem. We develop firmware that enables waveform readout of the silicon photomultipliers installed in the KLM detector. We employ a novel method for efficiently calibrating many channels at once, and we improve the hit- time granularity by a factor of four compared to the former firmware that did not use the waveform readout.
Link to LaTeX code https://gitlab.desy.de/cketter/dissertation.git

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