Study of Kaon Decay to Two Pions
  The weak decay of the kaon to two pions is studied within the NJL Model. Using the standard effective weak Hamiltonian, both the decay amplitude arising from an intermediate state \sigma-meson and the direct decay amplitude are calculated. The effect of final state interactions is also included. When the matching scale is chosen such that the decay amplitude with isospin I=2 is close to its experimental value, the \sigma-meson contributes more than 85% of the total I=0 amplitude. This supports recent suggestions that the \sigma-meson should play a vital role in explaining the \Delta I= 1/2 rule in this system. 
Zhan-Wei Liu, Manuel E. Carrillo-Serrano and Anthony W. Thomas
hep-ph