A measurement of the ratio of branching fractions of the decays B þ → K þ μ þ μ − and B þ → K þ e þ e − is presented. The proton-proton collision data used correspond to an integrated luminosity of 5.0 fb −1 recorded with the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. For the dilepton mass-squared range 1.1 < q 2 < 6.0 GeV 2 =c 4 the ratio of branching fractions is measured to be R K ¼ 0.846 þ0.060 −0.054 þ0.016 −0.014 , where the first uncertainty is statistical and the second systematic. This is the most precise measurement of R K to date and is compatible with the standard model at the level of 2.5 standard deviations.

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Searches are performed for both promptlike and long-lived dark photons, A 0 , produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A 0 → μ þ μ − decays using a data sample corresponding to an integrated luminosity of 5.5 fb −1 collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ-A 0 kinetic mixing strength. The promptlike A 0 search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214 < mðA 0 Þ ≲ 740 MeV and 10.6 < mðA 0 Þ ≲ 30 GeV. The search for long-lived A 0 → μ þ μ − decays places world-leading constraints on low-mass dark photons with lifetimes Oð1Þ ps.

An angular analysis of the B 0 → K Ã0 ð→ K þ π − Þμ þ μ − decay is presented using a dataset corresponding to an integrated luminosity of 4.7 fb −1 of pp collision data collected with the LHCb experiment. The full set of CP-averaged observables are determined in bins of the invariant mass squared of the dimuon system. Contamination from decays with the K þ π − system in an S-wave configuration is taken into account. The tension seen between the previous LHCb results and the standard model predictions persists with the new data. The precise value of the significance of this tension depends on the choice of theory nuisance parameters.