Johan Bijnens

We study the decays $K\to\pi\pi$ in one-loop
two-flavour Chiral Perturbation Theory.
We provide arguments why the calculation of the coefficient of the
pionic chiral logarithm
$\logm = M^2\log M^2$
is unique and then perform the calculation. As a check we perform the
reduction of the known three-flavour result.
Our result can be used to perform the extrapolation to the physical
pion mass of direct lattice QCD calculations of $K\to\pi\pi$
at fixed $m_s$ or $m_K^2$.
The underlying arguments
are expected to be valid for heavier particles and other processes as well.