K{yields}({pi}{pi}){sub I=2} decays and twisted boundary conditions

We propose a new method to evaluate the Lellouch-Luescher factor which relates the {Delta}I=3/2 K{yields}{pi}{pi} matrix elements computed on a finite lattice to the physical (infinite-volume) decay amplitudes. The method relies on the use of partially twisted boundary conditions, which allow the s-wave {pi}{pi} phase shift to be computed as an almost continuous function of the center-of-mass relative momentum and hence for its derivative to be evaluated. We successfully demonstrate the feasibility of the technique in an exploratory computation.