Direct CP violation in two-pion kaon decay (${\eta }_{+-}\simeq ϵ+{ϵ}^{\prime },{\eta }_{00}\simeq ϵ-2{ϵ}^{\prime }$)

Direct CP violation concerns the decay transition itself. At leading order, the charged and neutral two-pion decay amplitude ratios separate a common $ϵ$ from channel-dependent ${ϵ}^{\prime }$.^[ref]

For the corresponding double ratio of partial decay widths,The expansion follows from $(1-2x)/(1+x)=1-3x+O(x^2)$. A single nonzero long-lived two-pion yield does not isolate $x$: it can already occur with ${ϵ}^{\prime }=0$. Thus the 1964 evidence for CP violation must be distinguished from the later experimental establishment of direct CP violation through ${ϵ}^{\prime }$.