Is lepton flavor universality violated in $b\,\mathrm{to}\,c\,\tau\,\nu$ decays?
In plain words
The SM treats electrons, muons and taus alike except for mass. B meson decays to $\tau$ leptons occur more often than predicted, by more than 3 standard deviations.
Precise statement
Determine whether $R(D)=\operatorname{BR}(B \to D \tau \nu)/\operatorname{BR}(B \to D \ell \nu)$ and R(D*) exceed the SM values (HFLAV arithmetic average of predictions $0.296 \pm 0.004\ \text{and}\ 0.254 \pm 0.005$). The HFLAV ICHEP 2026 averages are $R(D)=0.359 \pm 0.024$ and R(D*) = 0.280 +- 0.011 (correlation -0.37), about $3.7\,\sigma$ above the SM combined; an answer is $\ge 5\,\sigma$ confirmation or reduction below $2\,\sigma$.
What would settle it
Belle II with several $\mathrm{ab}^{-1}$ and LHCb Run 3 measurements using hadronic and leptonic $\tau$ decays, with lattice form factors for $B \to D*$.
Status in the literature
Unverified note
The HFLAV average updated for ICHEP 2026 (August 2026, including a new Belle II $R(D*)$ result) gives about $3.7\sigma$.