Model-agnostic likelihood for the reinterpretation of the $\boldsymbol{B^{+}\to K^{+} \nu \bar{\nu}}$ measurement at Belle II

Sumitted to PubDB: 2024-12-07

Category: Physics / performance paper, Visibility: Latest Version Public

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Authors Lorenz Gaertner, Nikolai Krug, Thomas Kuhr, Slavomira Stefkova
Date 2025-11-26
Working group Radiative and Electroweak Penguin
Belle II Number BELLE2-PUB-PH-2024-038
Abstract We recently measured the branching fraction of the $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ decay using 362~fb$^{-1}$ of on-resonance $e^+e^-$ collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the Weak Effective Theory. Using a kinematic reweighting technique in combination with the published likelihood, we derive marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle~II data. For the Weak Effective Theory Wilson coefficients, the posterior mode of the magnitudes $|C_\mathrm{VL}+C_\mathrm{VR}|$, $|C_\mathrm{SL}+C_\mathrm{SR}|$, and $|C_\mathrm{TL}|$ corresponds to the point ${(11.3, 0.0, 8.2)}$. The respective 95\% credible intervals are $[1.9, 16.2]$, $[0.0, 15.4]$, and $[0.0, 11.2]$.
ArXiv https://arxiv.org/abs/2507.12393
Journal Phys.Rev.D
Preprint Number 2025-021
Authorship confirmation page https://authors.belle2.org/#88
Analysis [EWP-001] A new physics reinterpretation of the Belle II $B^{+} \to K^{+} \nu \bar \nu$ result with model-agnostic likelihoods

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