Branching fraction measurement of the $B^0\to K^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ decay at high dihadron mass

This Letter presents the first measurement of the $B^0\to K^{+}{\pi }^{-}{\mu }^{+}{\mu }^{-}$ differential branching fraction in the region of the dihadron $K^{+}{\pi }^{-}$ mass, $m_{K\pi }$, $1000<m_{K\pi }<1835\text{MeV}\mathrm{/}{c}^2$. This decay mode is mediated by the quark-level transition $b\to s{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$, for which the measured effective couplings show tensions with Standard Model predictions. The analysis is performed using proton-proton collision data collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of $8.4{\text{fb}}^{-1}$. The differential branching fraction is determined across different regions of dimuon mass squared, $q^2$, in the interval $0.1<q^2<18.0{\text{GeV}}^2\mathrm{/}{c}^4$, as well as in different regions of $m_{K\pi }$ in the interval $1.1<q^2<6.0{\text{GeV}}^2\mathrm{/}{c}^4$. The results represent the first differential measurement of the branching fraction across $m_{K\pi }$ within the selected dihadron-mass region, and the first observation of the decay within this region above the $J\mathrm{/}\psi$ resonance.