The Impact of |Delta I|=5/2 Transitions in K-> pi pi Decays
  We consider the impact of isospin violation on the analysis of K-> pi pi decays. We scrutinize, in particular, the phenomenological role played by the additional weak amplitude, of |Delta I|=5/2 in character, incurred by the presence of isospin violation. We show that Watson's theorem is appropriate in O(m_d-m_u), so that the inferred pi-pi phase shift at sqrt{s}=m_K determines the strong phase difference between the I=0 and I=2 amplitudes in K-> pi pi decay. We find the magnitude of the |Delta I|=5/2 amplitude thus implied by the empirical branching ratios to be larger than expected from estimates of isospin-violating strong and electromagnetic effects. We effect a new determination of the octet and 27-plet coupling constants with strong-interaction isospin violation and with electromagnetic effects, as computed by Cirigliano, Donoghue, and Golowich, and find that we are unable to resolve the difficulty. Exploring the role of |Delta I|=5/2 transitions in the CP-violating observable epsilon'/epsilon, we determine that the presence of a |Delta I|=5/2 amplitude impacts the empirical determination of omega, the ratio of the real parts of the |Delta I|=3/2 to |Delta I|=1/2 amplitudes, and that it generates a decrease in the estimation of epsilon'/epsilon. 
S. Gardner and G. Valencia