Test of lepton flavour universality with $B_s^0\to \phi {\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ decays

Lepton flavour universality in rare $b\to s$ transitions is tested for the first time using $B_s^0$ meson decays. The measurements are performed using $pp$ collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of 9${\mathrm{f}\mathrm{b}}^{-1}$. Branching fraction ratios between the $B_s^0\to \phi e^{+}{e}^{-}$ and $B_s^0\to \phi {\mu }^{+}{\mu }^{-}$ decays are measured in three regions of dilepton mass squared, $q^2$, with $0.1<q^2<1.1$, $1.1<q^2<6.0$, and $15<q^2<19{\mathrm{G}\mathrm{e}\mathrm{V}}^2\mathrm{/}{c}^4$. The results agree with the Standard Model expectation of lepton flavour universality.