Measuring the Breaking of Lepton Flavour Universality in $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$

Abstract

We propose measurements of weighted differences of the angular observables in the rare decays $B\to K^{\ast }{\ell }^{+}{\ell }^{-}$. The proposed observables are very sensitive to the difference between the Wilson coefficients ${\mathcal{C}}_9^{(e)}$ and ${\mathcal{C}}_9^{(\mu )}$ for decays into electrons and muons, respectively. At the same time, the charm-induced hadronic contributions are kinematically suppressed to $\lesssim 7\%(4\%)$ in the region $1$GeV${}^2\le q^2\le 6$GeV${}^2$, as long as LFU breaking occurs only in ${\mathcal{C}}_9^{(\ell )}$. This suppression becomes stronger for the region of low hadronic recoil, $q^2\ge 15$GeV${}^2$.

Comments: 8 pages, 5 figures