Optimized lepton universality tests in B → V ℓ ν ¯ decays

Abstract

We propose improved lepton flavor universality (LFU) ratios in semileptonic $P\to V\ell \overline{\nu }$ decays, when comparing $\mu$ and $\tau$ 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 $B_c\to J/\psi$ , $B_c\to \psi \left(2S\right)$ , $B\to D^{\ast }$ and $B_s\to D_s^{\ast }$ transitions, showing that in all cases we can reach $O(1\%)$ uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.