A non-zero value would violate the $\Delta \mathit S$ = $\Delta \mathit Q$ rule in $\mathit CPT$ conserving transitions. x$_{+}$ is defined above in the Re(x$_{-}$) section.
Determination of the Relative Decay Rate ${{\mathit K}_S^0}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit e}}{{\mathit \nu}}$/ ${{\mathit K}_L^0}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit e}}{{\mathit \nu}}$
AMBROSINO
2006E
PL B636 173
Study of the Branching Ratio and Charge Asymmetry for the Decay ${{\mathit K}_S^0}$ $\rightarrow$ ${{\mathit \pi}}{{\mathit e}}{{\mathit \nu}}$ with the KLOE Detector
ANGELOPOULOS
1998D
PL B444 38
Measurement of the ${{\mathit K}_L^0}$ $−$ ${{\mathit K}_S^0}$ Mass Difference using Semileptonic Decays of Tagged Neutral Kaons
Also
EPJ C22 55
$\mathit T$-Violation and $\mathit CPT$-Invariance Measurements in the CPLEAR Experiment: a Detailed Description of the Analysis of Neutral-Kaon Decays to ${{\mathit e}}$ ${{\mathit \pi}}$ ${{\mathit \nu}}$