Abstract
We discuss direct
CP
violation in the standard model by giving a new estimate of
ε
′
/ε
in kaon decays. Our analysis is based on the evaluation of the hadronic matrix elements of the \mbox{
ΔS=1
} effective quark lagrangian by means of the chiral quark model, with the inclusion of meson one-loop renormalization and NLO Wilson coefficients. Our estimate is fully consistent with the
ΔI=1/2
selection rule in
K→ππ
decays which is well reproduced within the same framework. By varying all parameters in the allowed ranges and, in particular, taking the quark condensate---which is the major source of uncertainty---between
(−200 MeV
)
3
and
(−280 MeV
)
3
we find
−5.0×
10
−3
<
ε
′
/ε< 1.4×
10
−3
.
Assuming for the quark condensate the improved PCAC result \mbox{$\vev{\bar qq} = -(221\: \pm 17\ {\rm MeV})^3$} and fixing
Λ
(4)
QCD
to its central value, we find the more restrictive prediction
ε
′
/ε=(4±5)×
10
−4
,
where the central value is defined as the average over the allowed values of Im
λ
t
in the first and second quadrants. In these estimates the relevant mixing parameter Im
λ
t
is self-consistently obtained from
ε
and we take
m
pole
t
=180±12
GeV. Our result is, to a very good approximation, renormalization-scale and
γ
5
-scheme independent.