title: K-->pipi amplitudes from lattice QCD with a light charm quark.
abstract: We compute the leading-order low-energy constants of the DeltaS=1 effective weak Hamiltonian in the quenched approximation of QCD with up, down, strange, and charm quarks degenerate and light. They are extracted by comparing the predictions of finite-volume chiral perturbation theory with lattice QCD computations of suitable correlation functions carried out with quark masses ranging from a few MeV up to half of the physical strange mass. We observe a DeltaI=1/2 enhancement in this corner of the parameter space of the theory. Although matching with the experimental result is not observed for the DeltaI=1/2 amplitude, our computation suggests large QCD contributions to the physical DeltaI=1/2 rule in the GIM limit, and represents the first step to quantify the role of the charm-quark mass in K-->pipi amplitudes. The use of fermions with an exact chiral symmetry is an essential ingredient in our computation.
authors.affiliation: Department of Physics, CERN, TH Division, CH-1211 Geneva 23, Switzerland.
authors.forename: L
authors.initials: L
authors.lastname: Giusti
authors.forename: P
authors.initials: P
authors.lastname: Hernández
authors.forename: M
authors.initials: M
authors.lastname: Laine
authors.forename: C
authors.initials: C
authors.lastname: Pena
authors.forename: J
authors.initials: J
authors.lastname: Wennekers
authors.forename: H
authors.initials: H
authors.lastname: Wittig