Theoretical status of ε ′/ε

Abstract

Abstract We briefly overview the historical controversy around Standard Model predictions of ε ′/ε and clarify the underlying physics. A full update of this important observable is presented, with all known short- and long-distance contributions, including isospin-breaking corrections. The current Standard Model prediction, Re(ε ′/ε) = (14 ± 5) · 10−4 [1, 2], is in excellent agreement with the experimentally measured value.