Matrix elements of electroweak penguin operators in the 1/Nc expansion

It is shown that the K→ππ matrix elements of the four-quark operator Q7, generated by the electroweak penguin-like diagrams of the Standard Model, can be calculated to first non-trivial order in the chiral expansion and in the 1/Nc expansion. Although the resulting B factors B7(1/2) and B7(3/2) are found to depend only logarithmically on the matching scale μ, their actual numerical values turn out to be rather sensitive to the precise choice of μ in the GeV region. We compare our results to recent numerical evaluations from lattice-QCD and to other model estimates.