Updated evaluation of $\varepsilon_K$ in the Standard Model with lattice QCD inputs

TL;DR

This paper reveals a significant 4-sigma discrepancy between the Standard Model prediction and experimental measurement of _K, highlighting tensions related to _K calculation inputs and the  determination methods.

Contribution

It provides an updated analysis of _K in the Standard Model using lattice QCD inputs, identifying a notable tension and its dependence on  determination methods.

Findings

Standard Model with lattice inputs accounts for only 70% of _K experimental value.

Discrepancy disappears when using inclusive  values from QCD sum rules.

Tension correlates with the difference between exclusive and inclusive  values.

Abstract

We report a strong tension in $\varepsilon_K$ at the $4\sigma$ level between the experimental value and the theoretical value calculated directly from the standard model using lattice QCD inputs such as $\hat{B}_K$, $|V_{cb}|$, $|V_{us}|$, $\xi_0$, $\xi_2$, $\xi_\text{LD}$, $F_K$, and $m_c$. The standard model with lattice QCD inputs describes only 70% of the experimental value of $\varepsilon_K$, and does not explain its remaining 30%. We also find that this tension disappears when we use the inclusive value of $|V_{cb}|$ (results of the heavy quark expansion based on QCD sum rules) to determine $\varepsilon_K$. This tension is highly correlated with the present discrepancy between the exclusive and inclusive values of $|V_{cb}|$. In order to resolve, in part, the issue with $|V_{cb}|$, it would be highly desirable to have a comprehensive re-analysis over the entire set of experimental data on the $\bar{B} \to D^* \ell \bar{\nu}$ decays using an alternative parametrization of the form factors, such as the BGL parametrization, and a comparison with results of the CLN method.