Remarks on the Standard Model predictions for $R(D)$ and $R(D^*)$

TL;DR

This paper examines the reliability of Standard Model predictions for the ratios R(D) and R(D*), highlighting the potential for new physics signals in R(D*) due to its robustness against theoretical uncertainties.

Contribution

The study analyzes the robustness of SM predictions for R(D) and R(D*), emphasizing R(D*) as a promising observable for new physics searches due to minimal theoretical pollution.

Findings

R(D) may be affected by lepton mass effects and scalar form factors.

R(D*) prediction is robust with negligible B* contribution pollution.

R(D*) is a promising observable for new physics detection.

Abstract

Semileptonic $b \to c$ transitions, and in particular the ratios $R(D^{(*)}) = \frac{\Gamma(B \rightarrow D^{(*)} \tau \nu )}{\Gamma(B \rightarrow D^{(*)} \ell \nu )}$, can be used to test the universality of the weak interactions. In light of the recent discrepancies between the experimental measurements of these observables by BaBar, Belle and LHCb and the Standard Model predicted values, we study the robustness of the latter. Our analysis reveals that $R(D)$ might be enhanced by lepton mass effects associated to the mostly unknown scalar form factor. In constrast, the Standard Model prediction for $R(D^*)$ is found to be more robust, since possible pollutions from $B^*$ contributions turn out to be negligibly small, which indicates that $R(D^*)$ is a promising observable for searches of new physics.