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

The introduction of partially twisted boundary conditions allows weak and electromagnetic form factors to be evaluated at specified values of the hadronic momenta (and hence momentum transfers) in lattice simulations. We present and demonstrate this technique for the computation of the K → π semileptonic form factor at zero momentum transfer and for the electromagnetic form factor of the pion at arbitrarily small momentum transfers. These exploratory computations are carried out in full QCD with 3 flavours of sea quarks, but with only two values of m_u = m_d which limits our ability to perform the chiral extrapolations. The results should therefore be viewed primarily as a demonstration of the feasibility of the method. For the K → π form factor we compare the new technique to the conventional approach and for the pion form factor we assess our results for very small momentum transfer with the help of chiral perturbation theory.