Lattice study of semileptonic form factors with twisted boundary conditions

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Abstract
We apply twisted boundary conditions to lattice QCD simulations of three-point correlation functions in order to access spatial components of hadronic momenta different from the integer multiples of 2 pi / L. We calculate the vector and scalar form factors relevant to the K -> pi semileptonic decay and consider all the possible ways of twisting one of the quark lines in the three-point functions. We show that the momentum shift produced by the twisted boundary conditions does not introduce any additional noise and easily allows to determine within a few percent statistical accuracy the form factors at quite small values of the four-momentum transfer, which are not accessible when periodic boundary conditions are considered. The use of twisted boundary conditions turns out to be crucial for a precise determination of the form factor at zero-momentum transfer, when a precise lattice point sufficiently close to zero-momentum transfer is not accessible with periodic boundary conditions.Comment: latex 15 pages, 4 figures and 3 tables; modified intro and discussions of the results; version to appear in PR