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

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 = 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_{− 0.23}^{+ 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 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.