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The combined analysis of the BaBar, Belle, and LHCb data on $B\to D\tau\nu$,$B\to D^*\tau\nu$ and $B_c\to J/\Psi\tau\nu$ decay observables shows evidenceof physics beyond the Standard Model (SM). In this article, we study all theone- and two-dimensional scenarios which can be generated by adding a singlenew particle to the SM. We put special emphasis on the model-discriminatingpower of $F_L(D^*)$ and of the $\tau$ polarizations, and especially on theconstraint from the branching fraction ${\rm BR}(B_c\to\tau\nu)$. We criticallyreview this constraint and do not support the aggressive limit of ${\rmBR}(B_c\to\tau\nu)<10\%$ used in some analyses. While the impact of $F_L(D^*)$is currently still limited, the ${\rm BR}(B_c\to\tau\nu)$ constraint has asignificant impact: depending on whether one uses a limit of $60\%$, $30\%$ or$10\%$, the pull for new physics (NP) in scalar operators changes drastically.More specifically, for a conservative $60\%$ limit a scenario with scalaroperators gives the best fit to data, while for an aggressive $10\%$ limit thisscenario is strongly disfavored and the best fit is obtained in a scenario inwhich only a left-handed vector operator is generated. We find a sum rule forthe branching ratios of $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $\Lambda_b\to\Lambda_c\tau\nu$ which holds for any NP contribution to the Wilsoncoefficients. This sum rule entails an enhancement of ${\rm BR}(\Lambda_b\to\Lambda_c\tau\nu)$ over its SM prediction by $(24\pm 6)\%$ for the current$\mathcal{R}(D^{(*)})$ data.

Impact of polarization observables and $ B_c\to \tau \nu$ on new physics explanations of the $b\to c \tau \nu$ anomaly