Search for $B^0 \to K_{\rm S}^0 \tau^+ \tau^-$ decays at Belle and Belle II

Sumitted to PubDB: 2026-10-05

Category: Phd Thesis

Tags: Belle Run 1

Principal Authors Ihor Prudiiev
Date 2026-10-05
Belle II Number BELLE2-PTHESIS-2026-025
Abstract We present the first search for the rare decay $B^{0}\!\to\!K_{\rm S}^0\tau^{+}\tau^{-}$ using approximately $1~\mathrm{ab}^{-1}$ of $e^+e^-$ collision data collected with the Belle and Belle~II experiments at the KEKB and SuperKEKB colliders between 1999--2009 and 2019--2022, respectively. The decay proceeds via a $b\!\to\! s\tau^{+}\tau^{-}$ flavor-changing neutral current transition, which is highly suppressed in the Standard Model and sensitive to possible contributions from new heavy mediators with preferential couplings to third-generation leptons. Since $\tau$ lepton decays involve neutrinos which escape detection, part of the decay information is inevitably lost, making the experimental reconstruction of the $B^{0}\!\to\!K_{\rm S}^0\tau^{+}\tau^{-}$ particularly challenging. We fully reconstruct the companion non-signal $\bar B^0$ meson in hadronic modes, enabling an exclusive study of the signal-side decay. The signal $B^{0}$ candidates are reconstructed in $\tau$ decay channels with a single charged particle, including leptonic and hadronic modes. A multivariate classifier is trained to enhance the discrimination between signal and background, and a simultaneous binned maximum-likelihood fit to the classifier output is performed across five orthogonal categories based on $\tau^+\tau^-$ decay modes for both Belle and Belle~II datasets. No significant excess above the expected background is observed. We set the only upper limit to date on the branching fraction, $\mathcal{B}(B^{0}\!\to\!K_{\rm S}^0\tau^{+}\tau^{-}) < 0.83\times10^{-3}$ at 90\% confidence level.
Institute Ljubljana(Uni-LJ)

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