Optimized lepton universality tests in [Formula: see text] decays

We propose improved lepton flavor universality (LFU) ratios in semileptonic [Formula: see text] decays, when comparing [Formula: see text] and [Formula: see text] 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 [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] transitions, showing that in all cases we can reach [Formula: see text] uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.