Branching fraction measurement of the $B^0\to K^+\pi^-\mu^+\mu^-$ decay at high dihadron mass
Abstract
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, $m_{K\pi}$, $1000<m_{K\pi}<1835\,\text{MeV}/c^{2}$. This decay mode is mediated by the quark-level transition $b \to s\ell^{+}\ell^{-}$, 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 $0.1<q^2<18.0\,\text{GeV}^2/c^{4}$, as well as in different regions of $ m_{K\pi}$ in the interval $1.1<q^2<6.0\,\text{GeV}^2/c^{4}$. 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 $J/\psi$ resonance.