New Probes of Electron--Muon Universality in $B\to K{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$ Decays

In the pursuit of physics beyond the Standard Model, a promising path is the study of B-meson decays caused by the transition $b\to s{\mathrm{\ell }}^{+}{\mathrm{\ell }}^{-}$. A key observable in such decays is the ratio $R_K$, which measures electron–muon universality in $B\to K{\mu }^{+}{\mu }^{-}\mathrm{/}{e}^{+}{e}^{-}$. At first sight, the recent LHCb measurement of $R_K\sim 1$ may seem to largely constrain deviations from universality in these decays. However, we show that this is actually not the case: new sources of CP violation allow for significant universality violation consistent with $R_K\sim 1$. This provides an exciting new opportunity to search for New Physics by measuring differences between CP asymmetries in $B\to K{\mu }^{+}{\mu }^{-}$ and $B\to K{e}^{+}{e}^{-}$.