This content will become publicly available on October 10, 2025
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Abstract A test of lepton flavor universality in
and decays, as well as a measurement of differential and integrated branching fractions of a nonresonant decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at 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 to is determined from the measured double ratio of these decays to the respective branching fractions of the with and decays, which allow for significant cancellation of systematic uncertainties. The ratio is measured in the range , whereq is the invariant mass of the lepton pair, and is found to be , in agreement with the standard model expectation . This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the sameq 2range, , is consistent with the present world-average value and has a comparable precision.