Searches for the B K^ (*) decays using an inclusive tagging method at the Belle II experiment

The rare decays B → Kν ¯ν represent a class of flavor-changing neutral current processes that are highlysuppressed in the Standard Model and thus serve as sensitive probes for new physics. These decays aretheoretically clean, with uncertainties primarily stemming from hadronic form factors, and their branchingfractions are predicted with high precision. However, their experimental study poses significant challengesdue to the presence of two undetectable neutrinos in the final state, necessitating sophisticated reconstruc-tion techniques and robust background suppression.This thesis investigates the decays B+ → K+ν ¯ν and B0 → K∗0ν ¯ν using data corresponding to 365 fb−1collected at the Υ(4S) resonance within the Belle II experiment. The analyses employ the inclusive taggingmethod, in which the companion B meson is reconstructed inclusively to infer the kinematics of the signal-side decay. This approach provides substantial gains in signal efficiency compared to exclusive tagging, albeitat the cost of increased background complexity.The branching fraction of B+ → K+ν ¯ν is extracted from a maximum likelihood fit, yielding 2.7±0.5(stat)±0.5(syst) × 10−5, which corresponds to a significance of 2.9 standard deviations from the Standard Modelexpectation. This measurement establishes the experimental viability of the inclusive tagging techniqueand has already prompted refinements of theoretical predictions, motivating renewed investigations intopotential new physics effects.Building upon this framework, the analysis of B0 → K∗0ν ¯ν is currently in the pre-unblinding stage. Thischannel poses additional challenges due to the more complex form-factor structure of the K∗0 meson andthe higher track multiplicity of its final states. Nevertheless, the analysis follows a similar strategy to theB+ → K+ν ¯ν case, with dedicated improvements in background modeling and event selection tailored tothe K∗0 topology. While no results are presented at this stage, the B0 → K∗0ν ¯ν study is expected to providecomplementary sensitivity to new physics through its unique helicity structure and kinematic features.In addition, this thesis includes a complementary study on a background filter in the central drift chamber,aimed at improving Belle II tracking performance under high-background conditions. While independent ofthe B → K(∗)ν ¯ν analyses, this work constitutes an independent contribution to optimization of the detectorperformance.