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Electroweak interactions assign a central role to the gauge group $SU(2)_L\times U(1)_Y$, which is either realized linearly (SMEFT) or nonlinearly (e.g.,HEFT) in the effective theory obtained when new physics above the electroweakscale is integrated out. Although the discovery of the Higgs boson has madeSMEFT the default assumption, nonlinear realization remains possible. The twocan be distinguished through their predictions for the size of certainlow-energy dimension-6 four-fermion operators: for these, HEFT predicts $O(1)$couplings, while in SMEFT they are suppressed by a factor $v^2/\Lambda_{\rmNP}^2$, where $v$ is the Higgs vev. One such operator, $O_V^{LR} \equiv ({\bar\tau} \gamma^\mu P_L \nu )\, ( {\bar c} \gamma_\mu P_R b )$, contributes to $b\to c \,\tau^- {\bar\nu}$. We show that present constraints permit itsnon-SMEFT coefficient to have a HEFTy size. We also note that the angulardistribution in ${\bar B} \to D^* (\to D \pi') \, \tau^{-} (\to \pi^- \nu_\tau){\bar\nu}_\tau$ contains enough information to extract the coefficients of allnew-physics operators. Future measurements of this angular distribution cantherefore tell us if non-SMEFT new physics is really necessary.

Beyond SMEFT with $b \to c \,\tau^- {\bar\nu}$