Optimized lepton universality tests in decays.

We propose improved lepton flavor universality (LFU) ratios in semileptonic decays, when comparing and modes, that minimize the theoretical form-factor uncertainties. These optimized ratios are obtained with simple cuts or reweighting of the dilepton mass distributions, which imply a minimum loss of signal on the rare tauonic modes while maximizing the cancellation of theoretical uncertainties among the two modes. We illustrate the usefulness of these observables in , , and transitions, showing that in all cases we can reach uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.