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 $\hat{I}\yen (4S)$ 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 $\mathrm{Br}(B^0\to K^{\ast }(892{)}^0(\to K^{+}\ddot{I}{€}^{-})S)\times \mathrm{Br}(S\to x^{+}{x}^{-})$ and $\mathrm{Br}(B^{+}\to K^{+}S)\times \mathrm{Br}(S\to x^{+}{x}^{-})$, where $x^{+}{x}^{-}$ indicates $e^{+}{e}^{-},\hat{I}{¼}^{+}\hat{I}{¼}^{-},\ddot{I}{€}^{+}\ddot{I}{€}^{-}$, or $K^{+}{K}^{-}$, as functions of $S$ mass and lifetime at the level of $10^{-7}$.