Test of lepton flavor universality inB±→K±μ+μ-andB±→K±e+e-decays in proton-proton collisions ats=13TeV.

A test of lepton flavor universality inB±→K±μ+μ-andB±→K±e+e-decays, as well as a measurement of differential and integrated branching fractions of a nonresonantB±→K±μ+μ-decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions ats=13TeVrecorded 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 fractionsB(B±→K±μ+μ-)toB(B±→K±e+e-)is determined from the measured double ratioR(K)of these decays to the respective branching fractions of theB±→J/ψK±withJ/ψ→μ+μ-ande+e-decays, which allow for significant cancellation of systematic uncertainties. The ratioR(K)is measured in the range1.1qis the invariant mass of the lepton pair, and is found to beR(K)=0.78-0.23+0.47, in agreement with the standard model expectationR(K)≈1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the sameq²range,B(B±→K±μ+μ-)=(12.42±0.68)×10-8, is consistent with the present world-average value and has a comparable precision.