# $R_{D^*}$ or $R_{D\pi}$: closing the theoretical gap?

## TL;DR

This paper revisits the theoretical predictions of $R_{D^*}$ and $R_{D o ext{pi}}$, clarifying previous discrepancies and confirming the tension with experimental results, which may hint at new physics beyond the standard model.

## Contribution

The study provides a corrected and detailed analysis of the full 4-body decay process, resolving previous numerical errors and aligning the predictions of $R_{D o ext{pi}}$ with $R_{D^*}$, while highlighting the remaining experimental-theoretical tension.

## Key findings

- Full 4-body decay analysis does not significantly alter predictions.
- Results on $R_{D o ext{pi}}$ are consistent with $R_{D^*}$.
- Tension with experimental data persists, suggesting possible new physics.

## Abstract

Measurements of the $R_{D^*}$ parameter remain in tension with the standard model prediction, despite recent results helping to close the gap. In this work, we revisit the standard model considerations for the prediction. We pay particular attention to the theoretical prediction considering the full 4-body decay $(B\rightarrow l\nu D^* \to l\nu D\pi)$, which introduces the longitudinal degree of freedom of the $D^*$. We show that this does not introduce sizeable effects at the current precision. This modifies our previous finding (Phys. Rev. D 98 056014 (2018)) where a numerical bug led us to a different conclusion. Thus, the results on $R_{D\pi}$ are consistent with $R_{D^*}$, and the difference between the several values can be traced back to the form factor used and the restrictions incorporated to determine their parameters. There is still tension between the experimental world average and the most accurate theoretical estimate, leaving the possibility of presence of new physics scenarios open.