Can the differences in the determinations of $V_{ub}$ and $V_{cb}$ be explained by New Physics?

TL;DR

This paper investigates whether new physics can explain the persistent discrepancies between inclusive and exclusive determinations of CKM elements $V_{cb}$ and $V_{ub}$, concluding that such explanations are currently ruled out.

Contribution

It provides a model-independent analysis showing that only a modified $W$ coupling could explain the discrepancies, but gauge invariance constraints rule out this possibility.

Findings

New physics cannot explain the $V_{cb}$ and $V_{ub}$ discrepancies due to gauge invariance constraints.

The discrepancies are likely due to underestimated uncertainties in measurements or theoretical calculations.

A modified $W$ coupling is the only operator that could potentially explain the differences, but it conflicts with experimental constraints.

Abstract

The precise determination of the CKM elements $V_{cb}$ and $V_{ub}$ is crucial for any new physics analysis in the flavour sector. Their values can be determined from several tree-level decays: $V_{cb}$ can be extracted from $B\to D\ell\nu$ and $B\to D^*\ell\nu$ while $V_{ub}$ can be obtained from $B\to \pi\ell\nu$, $B\to \rho\ell\nu$ and $B\to\tau\nu$. In addition, for both $V_{cb }$ and $V_{ub}$ an inclusive determination is available. There is a long lasting discrepancy between the inclusive and exclusive determinations which recently even increased for $V_{cb}$ above the $3\;\sigma$ level. In this article we study the possible effect of new physics on the inclusive and exclusive determination of $V_{cb}$ and $V_{ub}$ in a model independent way. We find that there is only one operator corresponding to a modified $W$ coupling which can achieve this. However, respecting $SU(2)$ gauge invariance at the high scale this would lead to very large violations of the $Z$ to $b\bar b$ coupling not compatible with experiment. Therefore, we conclude that a new physics explanation of the difference between the inclusive and exclusive determination of $V_{cb}$ and $V_{ub}$ is currently ruled out. Therefore, the discrepancies must be due underestimated uncertainties in the theoretical and/or the experimental analysis.