Scalar contributions to $b\to c (u) τν$ transitions

Abstract

We perform a comprehensive analysis of scalar contributions in $b \to c τν$ transitions including the latest measurements of $R(D^{(*)})$, the $q^2$ differential distributions in $B \to D^{(*)} τν$, the $τ$ polarization asymmetry for $B \to D^{*} τν$, 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 $R(D^{(*)})$ together with the measured differential distributions. However, the constraints from the total $B_c$ width present a slight tension with the current data on $B \to D^{*}τν$ in this scenario, preferring smaller values for $R(D^*)$. 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 $B \to D^{(*)}τν$ decays or additional decay modes like the baryonic $Λ_b \to Λ_c τν$ and the inclusive $B \to X_c τν$ decays. We also analyze scalar contributions in $b \to u τν$ transitions, including the latest measurements of $B \to τν$, providing predictions for $Λ_b \to p τν$ and $B \to πτν$ 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.

15 pages, 13 figures and 7 tables. More references and discussions added, Tab. VII updated; final version published in the journal