Test of Lepton Universality in $b\to s{\ell }^{+}{\ell }^{-}$ Decays

The first simultaneous test of muon-electron universality using $B^{+}\to K^{+}{\ell }^{+}{\ell }^{-}$ and $B^0\to K^{*0}{\ell }^{+}{\ell }^{-}$ 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 $9{\mathrm{fb}}^{-1}$. 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.

Figure 1

Variation of efficiency corrected ratios (top row, left to right): $r_{J/\psi }^K$, $R_{\psi (2S)}^K$, $R_K$ in low- and central-$q^2$ regions; (bottom row) analogous plots for the $K^{*0}$ channel. Measurements are relative to the final value for the given observable. The simulation calibration using $B^0$ and $B^{+}$ chains for successive calibration weights are shown and almost indistinguishable.

Figure 2

Invariant mass distributions of the nonresonant (left) $B^{+}\to K^{+}{e}^{+}{e}^{-}$ and (right) $B^0\to K^{*0}{e}^{+}{e}^{-}$ candidates in (upper) central-$q^2$ and (lower) low-$q^2$ regions. The results of the fit described in the text are also presented.