Search for $B^+ \to K^+ \tau^+ \tau^-$ decays at Belle II

Sumitted to PubDB: 2025-03-26

Category: Phd Thesis, Visibility: Latest Version Public

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Authors Debjit Ghosh, Diego Tonelli
Date 2025-01-23
Belle II Number BELLE2-PTHESIS-2025-005
Abstract This experimental particle-physics thesis reports on a search for the $B^{+} \to K^{+} \tau^{+} \tau^{-}$ rare decay. This decay, unobserved to date, is of particular interest for its sensitivity to potential contributions from non-Standard-Model amplitudes. The analysis uses the full data set of 387 million bottom-antibottom meson pairs from electron-positron collisions at the $\Upsilon(4S)$ collected by the Belle II experiment as of 2023. Belle II is an hermetic solenoidal magnetic spectrometer surrounded by particle-identification detectors, a calorimeter, and muon detectors installed at the KEK SuperKEKB collider. The search uses leptonic decays of the $\tau$ leptons, which offer the most sensitivity. I develop the analysis using simulated and control data and inspect the signal-search region only once all procedures are finalized. A restriction to the events in which the pair-produced nonsignal $B$ meson decays into fully reconstructed hadronic final-states suppresses significant backgrounds and enables inference of signal properties. In addition, an innovative restriction on the mass of the opposite-charge kaon-lepton pair suppresses dominant, and poorly modeled, residual background. An optimization of the signal selection further enhances sensitivity. I validate thoroughly the calorimeter energy unassociated with $\Upsilon(4S)$ reconstruction and determine the signal yield by counting the excess events over the expected background in its distribution. The analysis is currently under internal collaboration review and the signal-search region has not yet been undisclosed. The sensitivity of this first Belle II search is two times better than that of the world-leading result, in spite of relying on a smaller sample.

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