Searches for the $B\to K\nu\bar{\nu}$ decays using an inclusive tagging method at the Belle II experiment

Sumitted to PubDB: 2026-05-11

Category: Phd Thesis

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Principal Authors Yulan Fan
Date 2026-05-11
Belle II Number BELLE2-PTHESIS-2026-015
Abstract The rare decays $B\to K\nu\bar{\nu}$ represent a class of flavor-changing neutral current processes that are highly suppressed in the Standard Model and thus serve as sensitive probes for new physics. These decays are theoretically clean, with uncertainties primarily stemming from hadronic form factors, and their branching fractions are predicted with high precision. However, their experimental study poses significant challenges due to the presence of two undetectable neutrinos in the final state, necessitating sophisticated reconstruction techniques and robust background suppression. This thesis investigates the decays $B^+\to K^+\nu\bar{\nu}$ and $B^0\to K^{*0}\nu\bar{\nu}$ using data corresponding to $365~\mathrm{fb}^{-1}$ collected at the $\Upsilon(4S)$ resonance within the Belle~II experiment. The analyses employ the inclusive tagging method, 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, albeit at the cost of increased background complexity. The branching fraction of $B^+\to K^+\nu\bar{\nu}$ is extracted from a maximum likelihood fit, yielding $[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})] \times 10^{-5}$, which corresponds to a significance of 2.9 standard deviations from the Standard Model expectation. This measurement establishes the experimental viability of the inclusive tagging technique and has already prompted refinements of theoretical predictions, motivating renewed investigations into potential new physics effects. %The measurement of $B^+\to K^+\nu\bar{\nu}$ has already led to updated theoretical predictions, motivating renewed efforts to refine SM expectations and explore NP scenarios. Building upon this framework, the analysis of $B^0\to K^{*0}\nu\bar{\nu}$ is currently in the pre-unblinding stage. This channel poses additional challenges due to the more complex form-factor structure of the $K^{*0}$ meson and the higher track multiplicity of its final states. Nevertheless, the analysis follows a similar strategy to the $B^+\to K^+\nu\bar{\nu}$ case, with dedicated improvements in background modeling and event selection tailored to the $K^{*0}$ topology. While no results are presented at this stage, the $B^0\to K^{*0}\nu\bar{\nu}$ study is expected to provide complementary 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 of the $B\to K^{(*)}\nu\bar{\nu}$ analyses, this work constitutes an independent contribution to optimization of the detector performance.
Link to LaTeX code https://de.overleaf.com/read/rnshhdmxbqzx#39213a
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