New physics solutions for $R_D$ and $R_{D^*}$

TL;DR

Recent measurements of $R_{D^*}$ have lessened the discrepancy with the Standard Model, leading to constraints on new physics operators; only four solutions remain compatible with all data, emphasizing the importance of $ au$ lepton measurements.

Contribution

The paper identifies the limited set of new physics operators consistent with current $R_D$ and $R_{D^*}$ data, highlighting the necessity of $ au$ lepton measurements and the incompatibility of new physics only in $b ightarrow c\, ext{mu}\,ar{ u}$.

Findings

Most scalar/pseudoscalar solutions are ruled out by data.

Only four operator solutions remain viable.

New physics in $b ightarrow c\, ext{mu}\,ar{ u}$ alone is incompatible with all observables.

Abstract

Recent measurements of $R_{D^*}$ have reduced tension with the Standard Model prediction. Taking all the present data into account, we obtain the values of the Wilson coefficients of each new physics four-fermion operator of a given Lorentz structure. We find that the combined data rule out most of the solutions based on scalar/pseudoscalar operators. By studying the inter-relations between different solutions, we find that there are only four allowed solutions, which are based on operators with $(V-A)$, linear combination of $(V-A)$ and $(V+A)$, tensor and linear combination of scalar/pseudoscalar and tensor structure. We demonstrate that the need for new physics is driven by those measurement of $R_D$ and $R_{D^*}$ where the $\tau$ lepton is not studied. Further, we show that new physics only in $b\rightarrow c\,\mu\,\bar{\nu}$ is not compatible with the full set of observables in the decays $B\rightarrow Dl\bar{\nu}$ and $B\rightarrow D^*l\bar{\nu}$.