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

A test of lepton flavor universality inB±â†’K±µ+µ-andB±â†’K±e+e-decays, as well as a measurement of differential and integrated branching fractions of a nonresonantB±â†’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±â†’K±µ+µ-)toB(B±â†’K±e+e-)is determined from the measured double ratioR(K)of these decays to the respective branching fractions of theB±â†’J/ψK±withJ/ψ→µ+µ-ande+e-decays, which allow for significant cancellation of systematic uncertainties. The ratioR(K)is measured in the range1.1

Observation ofγγ→ττin proton-proton collisions and limits on the anomalous electromagnetic moments of the τ lepton.

The production of a pair of τ leptons via photon-photon fusion,γγ→ττ, is observed for the first time in proton-proton collisions, with a significance of 5.3 standard deviations. This observation is based on a data set recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb-1. Events with a pair of τ leptons produced via photon-photon fusion are selected by requiring them to be back-to-back in the azimuthal direction and to have a minimum number of charged hadrons associated with their production vertex. The τ leptons are reconstructed in their leptonic and hadronic decay modes. The measured fiducial cross section ofγγ→ττisσobsfid=12.4-3.1+3.8fb. Constraints are set on the contributions to the anomalous magnetic moment (aτ) and electric dipole moments (dτ) of the τ lepton originating from potential effects of new physics on theγττvertex:aτ=0.0009-0.0031+0.0032and|dτ|<2.9×10-17ecm(95% confidence level), consistent with the standard model.