Standard Model evaluation of $\varepsilon_K$ using lattice QCD inputs for $\hat{B}_K$ and $V_{cb}$

TL;DR

This paper evaluates the Standard Model prediction of the CP violation parameter $oldsymbol{ ext{epsilon}_K}$ using lattice QCD inputs for $oldsymbol{ ext{B}_K}$ and $oldsymbol{V_{cb}}$, revealing a discrepancy with experiment depending on the $V_{cb}$ determination.

Contribution

It provides a comprehensive Standard Model calculation of $ ext{epsilon}_K$ incorporating lattice QCD inputs and compares predictions using exclusive and inclusive $V_{cb}$ values.

Findings

Exclusive $V_{cb}$ leads to a 3.4$\sigma$ deficit in $ ext{epsilon}_K$ prediction.

Inclusive $V_{cb}$ aligns the $ ext{epsilon}_K$ prediction with experimental data.

Calculated $ ext{eta}_{cc}$ at NNLO as $ ext{1.72(27)}$.

Abstract

We report the Standard Model evaluation of the indirect CP violation parameter $\varepsilon_K$ using inputs determined from lattice QCD: the kaon bag parameter $\hat{B}_K$, $\xi_0$, $|V_{us}|$ from the $K_{\ell 3}$ and $K_{\mu 2}$ decays, and $|V_{cb}|$ from the axial current form factor for the exclusive decay $\bar{B} \to D^* \ell \bar{\nu}$ at zero-recoil. The theoretical expression for $\varepsilon_K$ is thoroughly reviewed to give an estimate of the size of the neglected corrections, including long distance effects. The Wolfenstein parametrization $(|V_{cb}|, \lambda, \bar{\rho}, \bar{\eta})$ is adopted for CKM matrix elements which enter through the short distance contribution of the box diagrams. For the central value, we take the Unitarity Triangle apex $(\bar{\rho}, \bar{\eta})$ from the angle-only fit of the UTfit collaboration and use $V_{us}$ as an independent input to fix $\lambda$. We find that the Standard Model prediction of $\varepsilon_K$ with exclusive $V_{cb}$ (lattice QCD results) is lower than the experimental value by $3.4\sigma$. However, with inclusive $V_{cb}$ (results of the heavy quark expansion), there is no gap between the Standard Model prediction of $\varepsilon_K$ and its experimental value. For the calculation of $\varepsilon_K$, we perform the renormalization group running to obtain $\eta_{cc}$ at next-to-next-to-leading-order; we find $\eta_{cc}^\mathrm{NNLO}=1.72(27)$.