ENHANCEMENT OF THE K(L)-K(S) MASS DIFFERENCE BY SHORT-DISTANCE QCD CORRECTIONS BEYOND LEADING LOGARITHMS

We calculate the next-to-leading order short-distance QCD corrections to the coefficient eta1 of the effective DELTAS = 2 hamiltonian in the standard model. This part dominates the short-distance contribution ( DELTAm(K))SD to the K(L)-K(S) mass difference. The next-to-leading ord er result enhances eta1 and (DELTAm(K))SD by 20% compared to the leadi ng order estimate. Taking 0.200 GeV less-than-or-equal-to LAMBDA(MSBAR ) less-than-or-equal-to 0.350 GeV and 1.35 GeV less-than-or-equal-to m (c)(m(c)) less-than-or-equal-to 1.45 GeV we obtain 0.922 less-than-or- equal-to eta1NLO less-than-or-equal-to 1.419 compared to 0.834 eta1LO less-than-or-equal-to 1.138. For B(K) = 0.7 this corresponds to (48-75 )% of the experimentally observed mass difference. The inclusion of ne xt-to-leading order corrections to eta1 reduces considerably the theor etical uncertainty related to the choice of renormalization scales.