Search for a long-lived spin-0 mediator in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>b</mml:mi><mml:mo stretchy="false">→</mml:mo><mml:mi>s</mml:mi></mml:math> 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 $b\ensuremath{\rightarrow}s$ quark transition in ${e}^{+}{e}^{\ensuremath{-}}$ collisions at the $\mathrm{\ensuremath{\Upsilon}}(4S)$ resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of $189\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{K}^{*}(892{)}^{0}(\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})S)\ifmmode\times\else\texttimes\fi{}\mathcal{B}(S\ensuremath{\rightarrow}{x}^{+}{x}^{\ensuremath{-}})$ and $\mathcal{B}({B}^{+}\ensuremath{\rightarrow}{K}^{+}S)\ifmmode\times\else\texttimes\fi{}\mathcal{B}(S\ensuremath{\rightarrow}{x}^{+}{x}^{\ensuremath{-}})$, where ${x}^{+}{x}^{\ensuremath{-}}$ indicates ${e}^{+}{e}^{\ensuremath{-}},{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}},{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, or ${K}^{+}{K}^{\ensuremath{-}}$, as functions of $S$ mass and lifetime at the level of ${10}^{\ensuremath{-}7}$.