Optimized lepton universality tests in $$B\rightarrow V \ell {\bar{\nu }}$$ decays

We propose improved lepton flavor universality (LFU) ratios in semileptonic $$P\rightarrow V \ell {\bar{\nu }}$$ P → V ℓ ν ¯ decays, when comparing $$\mu $$ μ and $$\tau $$ τ 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 $$B_c\rightarrow J/\psi $$ B c → J / ψ , $$B_c\rightarrow \psi (2S)$$ B c → ψ ( 2 S ) , $$B\rightarrow D^*$$ B → D ∗ and $$B_s\rightarrow D_s^*$$ B s → D s ∗ transitions, showing that in all cases we can reach $${\mathcal {O}}(1\%)$$ O ( 1 % ) uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.