Test of lepton flavour universality with $B^0\to K^{\ast 0}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays at large dilepton invariant mass

Muon-electron universality is tested in $B^0\to K^{\ast 0}{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays, in the dilepton-invariant-mass region above the $\psi (2S)$ resonance. The analysis uses beauty mesons produced in proton-proton collisions recorded by the LHCb detector at center-of-mass energies of 7, 8, and 13 $\text{TeV}$, corresponding to an integrated luminosity of 9 ${\text{fb}}^{-1}$. The ratio of branching fractions between the muon and electron channels, $R_{K^{\ast 0}}$, is measured to be $1.08{}_{-0.12}^{+0.14}\text{(stat) }\pm 0.07\text{(syst)}$ for a dilepton-invariant-mass squared above 14.0 ${\text{GeV}}^2\mathrm{/}{\text{c}}^4$, consistent with the Standard Model prediction. This result represents the most precise measurement of $R_{K^{\ast 0}}$ in this region and the first such measurement performed at a hadron collider.