Test of lepton flavor universality in B${}^{\pm }$ $\to$ K${}^{\pm }{\mu }^{+}{\mu }^{-}$ and B${}^{\pm }$ $\to$ K${}^{\pm }$e${}^{+}$e${}^{-}$ decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract

A test of lepton flavor universality in B${}^{\pm }$ $\to$ K${}^{\pm }{\mu }^{+}{\mu }^{-}$ and B${}^{\pm }$ $\to$ K${}^{\pm }$e${}^{+}$e${}^{-}$ decays, as well as a measurement of differential and integrated branching fractions of a nonresonant B${}^{\pm }$ $\to$ K${}^{\pm }{\mu }^{+}{\mu }^{-}$ decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at $\sqrt{s}$ = 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 $\mathcal{B}$(B${}^{\pm }$ $\to$ K${}^{\pm }{\mu }^{+}{\mu }^{-}$) to $\mathcal{B}$(B${}^{\pm }$ $\to$ K${}^{\pm }$e${}^{+}$e${}^{-}$) is determined from the measured double ratio $R$(K) of these decays to the respective branching fractions of the B${}^{\pm }$ $\to$ J/$\psi$K${}^{\pm }$ with J/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}$ and e${}^{+}$e${}^{-}$ decays, which allow for significant cancellation of systematic uncertainties. The ratio $R$(K) is measured in the range 1.1 $<$ $q^2$ $<$ 6.0 GeV${}^2$, where $q$ is the invariant mass of the lepton pair, and is found to be $R$(K) = 0.78 ${}_{-0.23}^{+0.47}$, in agreement with the standard model expectation $R$(K) $\approx$ 1. This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the same $q^2$ range, $\mathcal{B}$(B${}^{\pm }$ $\to$ K${}^{\pm }{\mu }^{+}{\mu }^{-}$) = (12.42 $\pm$ 0.68) $\times$ 10${}^{-8}$, is consistent with the present world-average value and has a comparable precision.

arXiv commentReplaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/BPH-22-005 (CMS Public Pages)

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