Search for a long-lived spin-0 mediator in b→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→s quark transition in e⁺e^- collisions at the Υ(4S) resonance at the Belle II experiment. Based on a sample corresponding to an integrated luminosity of 189 fb^-1, we observe no evidence for signal. We set model-independent upper limits on the product of branching fractions $B({B}⁰{→}{K}*(892{)}⁰({→}{K}⁺{{π}}^{{-}})S)×{}B(S{→}{x}⁺{x}^{{-}})$ and $B({B}⁺{→}{K}⁺S)×{}B(S{→}{x}⁺{x}^{{-}})$, where ${x}⁺{x}^{{-}}$ indicates ${e}⁺{e}^{{-}},{{μ}}⁺{{μ}}^{{-}},{{π}}⁺{{π}}^{{-}}$, or ${K}⁺{K}^{{-}}$, as functions of $S$ mass and lifetime at the level of ${10}^{{-}7}$.