Test of lepton flavor universality in $\mathrm{B}^{\pm}\rightarrow \mathrm{K}^\pm \ell^+ \ell^-$ decays

A test of lepton flavor universality in $\mathrm{B}^\pm \to \mathrm{K}^\pm\ell^+\ell^-$ decays, where $\ell$ is a muon or electron, as well as a measurement of differential and inclusive branching fractions of a nonresonant $\mathrm{B}^\pm \to\mathrm{K}^\pm\mu^+\mu^-$ decay with the CMS experiment at the LHC are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at $\sqrt{s} = 13$ TeV recorded in 2018, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions ${\cal B}(\mathrm{B}^\pm \to \mathrm{K}^\pm\mu^+\mu^-$) to ${\cal B}(\mathrm{B}^\pm \to \mathrm{K}^\pm\mathrm{e^+e^-}$) is measured as a double ratio $R(\mathrm{K})$ of these decays to the respective branching fractions of the $\mathrm{B}^\pm \to \mathrm{J}/\psi\mathrm{K}^\pm$ ($\mathrm{J}/\psi \to \mu^+\mu^-)$ and ($\mathrm{J}/\psi\to \mathrm{e^+e^-})$ decays, which allow for significant cancellation of systematic uncertainties. The ratio $R(\mathrm{K})$ is measured in a range $1.1 < q^2 < 6.0$ GeV$^2$, where $q$ is the invariant mass of the lepton pair, and is found to be $R({\rm K})=0.78^{+0.47}_{-0.23}$, in agreement with the standard model expectation within one standard deviation. This measurement is limited by the statistical precision of the electron channel. The inclusive branching fraction in the same $q^2$ range of ${\cal B}(\mathrm{B}^\pm \to \mathrm{K}^\pm\mu^+\mu^-) = (12.42 \pm 0.68)\times 10^{-8}$ is consistent with and has a comparable precision to the present world average value.