$\vert \eta _{000}\vert $ = $\vert \boldsymbol A$( ${{\boldsymbol K}_S^0}$ $\rightarrow$ 3 ${{\boldsymbol \pi}^{0}}$ )/$\boldsymbol A$( ${{\boldsymbol K}_L^0}$ $\rightarrow$ 3 ${{\boldsymbol \pi}^{0}}$ )$\vert $
INSPIRE search
A non-zero value violates CP invariance.