Test of lepton flavor universality in B±→K±μ+μ− and B±→K±e+e− decays in proton-proton collisions at s=13TeV

Abstract

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=13TeV 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<6.0GeV2 , where q is the invariant mass of the lepton pair, and is found to be R(K)=0.78−0.23+0.47 , 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 q 2 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.