Search for a long-lived spin-0 mediator in $b\to s$ transitions at the Belle II experiment

Abstract

Additional spin-0 particles appear in many extensions of the standard model. We search for long-lived spin-0 particles $S$ in $B$-meson decays mediated by a $b\to s$ quark transition in $e^{+}{e}^{-}$ collisions at the ${\rm Y}(4S)$ resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of $189{\mathrm{fb}}^{-1}$, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $\mathrm{Br}(B^0\to K^{\ast }(892{)}^0(\to K^{+}{\pi }^{-})S)\times \mathrm{Br}(S\to x^{+}{x}^{-})$ and $\mathrm{Br}(B^{+}\to K^{+}S)\times \mathrm{Br}(S\to x^{+}{x}^{-})$, where $x^{+}{x}^{-}$ indicates $e^{+}{e}^{-},{\mu }^{+}{\mu }^{-},{\pi }^{+}{\pi }^{-}$, or $K^{+}{K}^{-}$, as functions of $S$ mass and lifetime at the level of $10^{-7}$.

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• Long-lived Light Mediators in a Higgs Portal Model at the FCC-ee
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• Di-decay signature of new physics particles at intensity frontier experiments