Semi-inclusive Lepton Flavor Universality ratio in $b\to s{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ transitions

We construct a semi-inclusive Lepton Flavour Universality (LFU) ratio, $R_{\mathrm{\Sigma }}$, to test $\mu \mathrm{/}e$ universality in $b\to s{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ transitions at $e^{+}{e}^{-}$ $B$-meson factories. Combining different decay channels, this observable maximises the sensitivity to possible LFU violations of short distance origin, yet preserving a clean theoretical interpretation in case of a deviation from its Standard Model prediction, $R_{\mathrm{\Sigma }}^{\mathrm{S}\mathrm{M}}=1$.