Contribution of the charm quark to the \Delta I=1/2 rule
  We report on the progress of our ongoing project to quantify the role of the charm quark in the non-leptonic decay of a kaon into two pions. The effect of its associated mass scale in the dynamics underlying the \Delta I = 1/2 rule can be studied by monitoring the dependence of kaon decay amplitudes on the charm quark mass using an effective \Delta S = 1 weak Hamiltonian. In contrast to commonly used approaches the charm quark is kept as an active degree of freedom. Quenched results in the GIM limit have shown that a significant part of the \Delta I = 1/2 enhancement is purely due to low-energy QCD effects. Moving away from the GIM limit involves the computation of diagrams containing closed quark loops which requires new variance reduction techniques in order to determine the relevant weak effective low-energy couplings. We employ a combination of low-mode averaging and stochastic volume sources in order to compute these diagrams and observe a significant improvement in the statistical signal. 
Eric Endress, Carlos Pena
hep-lat