Probing dark-matter scenarios of Belle II excess in $B^+\to K^+M_X$ with $K^+\to π^+ M_X$ decays

Abstract

The hypothesis of top-philic dark matter as the origin of the Belle II excess events in $B\to K M_X$ decays has direct impact on $K\to πM_X$ decays: Both of these decays proceed via loop diagrams~with a top quark emitting some mediator field $R$ and the subsequent decay of $R$ into a pair of dark-matter fermions $\barχχ$. The Belle II results on $B\to K M_X$ allowed us to extract the parameters of the mediator $R$ with reasonable accuracy, whereas the sensitivity to the fermion mass $m_χ$ proved to be rather low. In contrast to $B\to K M_X$ decays, the $K\to πM_X$ decays exhibit a strong sensitivity to $m_χ$; therefore, if - upon reducing theoretical and experimental uncertainties - the tension between standard-model predictions for the $K\toπM_X$ decays and the most recent NA62 measurements becomes pronounced the latter decays may either enable the extraction of the value of $m_χ$ or provide a lower limit on~$m_χ$.