Hadronic form factors in lattice QCD at small and vanishing momentum transfer

The introduction of partially twisted boundary conditions allows weak andelectromagnetic form factors to be evaluated at specified values of thehadronic momenta (and hence momentum transfers) in lattice simulations. Wepresent and demonstrate this technique for the computation of the K->pisemileptonic form factor at zero momentum transfer and for the electromagneticform factor of the pion at arbitrarily small momentum transfers. Theseexploratory computations are carried out in full QCD with 3 flavours of seaquarks, but with only two values of m_u=m_d which limits our ability to performthe chiral extrapolations. The results should therefore be viewed primarily asa demonstration of the feasibility of the method. For the K-> pi form factor wecompare the new technique to the conventional approach and for the pion formfactor we assess our results for very small momentum transfer with the help ofchiral perturbation theory.Comment: 19 pages, 6 figure