Direct CP violation in $K^0\to\pi\pi$: Standard Model Status

TL;DR

This paper reviews the status of direct CP violation in $K^0 o ext{pi} ext{pi}$ decays within the Standard Model, discussing theoretical predictions, recent lattice data, and comparing with experimental results, highlighting ongoing debates and future prospects.

Contribution

It provides an updated Standard Model prediction for $ ext{Re}(rac{ ext{epsilon}'}{ ext{epsilon}})$, incorporating recent lattice data and analyzing past discrepancies in theoretical estimates.

Findings

Standard Model prediction: (15 ± 7) × 10^{-4} for Re(ε'/ε)

Good agreement between theoretical prediction and experimental measurement

Revived debate due to improved lattice QCD calculations

Abstract

In 1988 the NA31 experiment presented the first evidence of direct CP violation in the $K^0\to\pi\pi$ decay amplitudes. A clear signal with a $7.2\,\sigma$ statistical significance was later established with the full data samples from the NA31, E731, NA48 and KTeV experiments, confirming that CP violation is associated with a $\Delta S=1$ quark transition, as predicted by the Standard Model. However, the theoretical prediction for the measured ratio $\varepsilon'/\varepsilon$ has been a subject of strong controversy along the years. Although the underlying physics was already clarified in 2001, the recent release of improved lattice data has revived again the theoretical debate. We review the current status, discussing in detail the different ingredients that enter into the calculation of this observable and the reasons why seemingly contradictory predictions were obtained in the past by several groups. An update of the Standard Model prediction is presented and the prospects for future improvements are analysed. Taking into account all known short-distance and long-distance contributions, one obtains $\mbox{Re}\left(\varepsilon'/\varepsilon\right) = (15 \pm 7)\cdot 10^{-4}$, in good agreement with the experimental measurement.