Measuring the Breaking of Lepton Flavour Universality in $Bto K^*ell^+ell^-$

We propose measurements of weighted differences of the angular observables in the rare decays $B to K^*ell^+ell^-$. 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$^2 leq q^2 leq 6,$GeV$^2$, as long as LFU breaking occurs only in $mathcal{C}^{(ell)}_{9}$. This suppression becomes stronger for the region of low hadronic recoil, $q^2 geq 15,$GeV$^2$.