STATUS OF THE $\Delta I =\frac{1}{2}$ RULE IN KAON DECAY

Abstract

The [Formula: see text] rule in K → ππ decays and its status are discussed and reviewed. First, we present the phenomenological deductions which have a firm basis, including the penguin diagram, vacuum insertion, current algebra and chiral perturbation theory. Then we proceed to discuss new conjectures and speculations e.g. 1/N expansion, W-exchange, low-energy penguin diagrams, the s - d self-energy tadpole, QCD-duality, instanton effects, etc. and to assess their roles in understanding the [Formula: see text] enigma.