A test of lepton flavor universality in B± → K±μ+μ− and B± → K±e+e− decays, as well as a measurement of differential and integrated branching fractions of a nonresonant B± → K±μ+μ− decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at √s = 13 TeV recorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions B(B± → K±μ+μ−) to B(B± → K±e+e−) is determined from the measured double ratio R(K) of these decays to the respective branching fractions of the B± → J/ψK± with J/ψ → μ+μ− and e+e− decays, which allow for significant cancellation of systematic uncertainties. The ratio R(K) is measured in the range 1.1 < q2 < 6.0 GeV2, where q is the invariant mass of the lepton pair, and is found to be R(K) = 0.78+0.47−0.23, in agreement with the standard model expectation R(K) ≈ 1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the same q2 range, B(B± → K±μ+μ−) = (12.42 ± 0.68) × 10−8, is
consistent with the present world-average value and has a comparable precision.
