Scalar contributions to $b\to c(u)\tau \nu$ transitions

We perform a comprehensive analysis of scalar contributions in $b \to c \tau \nu$transitionsincludingthelatestmeasurementsof$R(D^{()})$,the$q^2$differentialdistributionsin$B \to D^{()} \tau \nu$,the$\tau$ polarization asymmetry for $B\to D^{\ast }\tau \nu$, and the bound derived from the total width of the $B_c$ meson. We find that scalar contributions with the simultaneous presence of both left- and right-handed couplings to quarks can explain the available data, specifically $R(D^{(\ast )})$ together with the measured differential distributions. However, the constraints from the total $B_c$ width present a slight tension with the current data on $B \to D^{}\tau \nu$inthisscenario,preferringsmallervaluesfor$R(D^)$. We discuss possibilities to disentangle scalar new physics from other new-physics scenarios like the presence of only a left-handed vector current, via additional observables in $B\to D^{(\ast )}\tau \nu$ decays or additional decay modes like the baryonic ${\mathrm{\Lambda }}_b\to {\mathrm{\Lambda }}_c\tau \nu$ and the inclusive $B \to X_c \tau \nu$decays\mathrm{.}Wealsoanalyzescalarcontributionsin$b \to u \tau \nu$transitions,includingthelatestmeasurementsof$B \to \tau \nu$, providing predictions for ${\mathrm{\Lambda }}_b\to p\tau \nu$ and $B\to \pi \tau \nu$ decays. The potential complementarity between the $b\to u$ and $b\to c$ sectors is finally investigated once assumptions about the flavour structure of the underlying theory are made.