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

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 $\mathrm{{\rm Y}}(4S)$ resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of $189{\mathrm{f}\mathrm{b}}^{-1}$, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $\mathrm{B}\mathrm{r}(B^0\to K^{\ast }(892{)}^0(\to K^{+}{\pi }^{-})S)\times \mathrm{B}\mathrm{r}(S\to x^{+}{x}^{-})$ and $\mathrm{B}\mathrm{r}(B^{+}\to K^{+}S)\times \mathrm{B}\mathrm{r}(S\to x^{+}{x}^{-})$, where $x^{+}{x}^{-}$ indicates $e^{+}{e}^{-},{\mu }^{+}{\mu }^{-},{\pi }^{+}{\pi }^{-}$, or $K^{+}{K}^{-}$, as functions of $S$ mass and lifetime at the level of ${10}^{-7}$.