Tensions and correlations in $|V_{cb}|$ determinations

TL;DR

This paper analyzes recent determinations of |V_{cb}| from exclusive B decays, finds the tensions are less than 3σ when correlations are considered, and highlights the role of heavy quark symmetry violations and lattice QCD data in these discrepancies.

Contribution

It provides a detailed correlation analysis of |V_{cb}| fits, reducing the perceived tension and examining the impact of theoretical assumptions and lattice data on the results.

Findings

Tension between |V_{cb}| determinations is less than 3σ when correlations are included.

Higher |V_{cb}| fits suggest large violations of heavy quark symmetry.

Current data and lattice results do not resolve the |V_{cb}| discrepancy.

Abstract

Recently several papers extracted $|V_{cb}|$ using the Belle measurement of the exclusive $\bar B \to D^* \ell\bar\nu$ unfolded differential decay rates, available for the first time. Depending on the theoretical inputs, some of the fits yield higher $|V_{cb}|$ values, compatible with those from inclusive semileptonic $B$ decays. Since these four fits use mostly the same data, if their correlations were close to 100\%, the tension between them would be over $5\sigma$. We determine the correlations, find that the tension between the results is less than $3\sigma$, and explore what might lead to improving the consistency of the fits. We find that fits that yield the higher values of $|V_{cb}|$, also suggest large violations of heavy quark symmetry. These fits are also in tension with preliminary lattice QCD data on the form factors. Without additional experimental data or lattice QCD input, there are no set of assumptions under which the tension between exclusive and inclusive determinations of $|V_{cb}|$ can be considered resolved.