PoS - Proceedings of Science
Volume 449 - The European Physical Society Conference on High Energy Physics (EPS-HEP2023) - T10 Searches for New Physics
Search for charged lepton flavor violation at Belle
S. Patra* and  On behalf of the BELLE Collaboration
Full text: pdf
Pre-published on: January 30, 2024
Published on: March 21, 2024
We report the results of recent searches on charged lepton flavor violations using the $e^+e^-$ collision data collected by the Belle detector. In the first section, we present a search for the lepton-flavor-violating decays $B{}^0_s \rightarrow \ell^{\mp}\tau^{\pm}$, where $\ell = e, \mu$, using $121~\mathrm{fb}^{-1}$ data collected at the $\Upsilon(5S)$ resonance. This analysis uses semi-leptonic tagging method. We set upper limits on their branching fractions at $90\%$ confidence level (CL) as $\mathcal{B}(B{}^0_s \rightarrow e^{\mp}\tau^{\pm}) < 14.1 \times 10^{-4}$ and $\mathcal{B}(B{}^0_s \rightarrow \mu^{\mp}\tau^{\pm}) < 7.3 \times 10^{-4}$. Our result represents the first upper limit on the $B{}^0_s \rightarrow e^{\mp}\tau^{\pm}$ decay rate. We also search for the lepton-flavour-violating decays $B^+ \to K^+ \tau^\pm \ell^\mp$, with~$\ell = (e, \mu)$, using $711~{\rm fb}^{-1}$ $\Upsilon(4S)$ data sample. We use events where one $B$ meson is fully reconstructed in a hadronic decay mode. We find no evidence for $B^+ \to K^+ \tau^\pm \ell^\mp$ decays and set upper limits on their branching fractions at the 90\% CL in the $(1$--$3) \times 10^{-5}$ range. The obtained limits are the world's best results. Finally, we
present a search for the charged lepton-flavor-violating decays $\Upsilon(1S)\rightarrow\ell^\pm\ell^{\prime\mp}$ and radiative charged lepton-flavour-violating decays $\Upsilon(1S)\rightarrow\gamma\ell^\pm\ell^{\prime\mp}$ [$\ell,\ell^\prime = e, \mu, \tau$] using $25~{\rm fb}^{-1}$ data recorded at $\Upsilon(2S)$ resonance. This search uses $\Upsilon(1S)$ mesons produced in $\Upsilon(2S)\rightarrow\pi^+\pi^-\Upsilon(1S)$ transitions. In the absence of significant signal events, we provide upper limits on the branching fractions at the 90\% CL. This analysis provides the most stringent upper limits for all the decay channels.
DOI: https://doi.org/10.22323/1.449.0466
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