Test of lepton universality in $b\to s{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays

The first simultaneous test of muon-electron universality using $B^{+}\to K^{+}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ and $B^0\to K^{\ast 0}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays is performed, in two ranges of the dilepton invariant-mass squared, $q^2$. The analysis uses beauty mesons produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 ${\mathrm{f}\mathrm{b}}^{-1}$. Each of the four lepton universality measurements reported is either the first in the given $q^2$ interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.