$\varepsilon'/\varepsilon$ in the Standard Model and Beyond: 2021

Abstract

The present status of the ratio $\varepsilon'/\varepsilon$ in the Standard Model (SM) is summarized. In particular we stress the differences between three approaches that attempt to calculate $\varepsilon'/\varepsilon$ within the SM: Lattice QCD, Dual QCD and Chiral Perturbation Theory. As presently there is still a significant room left for new physics (NP) contributions, we summarize the present status of the technology that has been recently developed with the goal to analyze $\varepsilon'/\varepsilon$ and its correlations with other observables within the Weak Effective Theory (WET) and the Standard Model Effective Field Theory (SMEFT). We also make a few comments on the interpretation of the main dynamics behind the $\Delta I=1/2$ rule in $K\to\pi\pi$ decays.