Twisted finite-volume corrections to K_l3 decays with partially-quenched and rooted-staggered quarks

The determination of |V_us| from kaon semileptonic decays requires the value of the form factor f₊(q² = 0) which can be calculated precisely on the lattice. We provide the one-loop partially quenched chiral perturbation theory expressions both with and without including the effects of staggered quarks for all form factors at finite volume and with partially twisted boundary conditions for both the vector current and scalar density matrix elements at all q². We point out that at finite volume there are more form factors than just f₊ and f₋ for the vector current matrix element but that the Ward identity is fully satisfied. The size of the finite-volume corrections at present lattice sizes is small. This will help improve the lattice determination of f₊(q² = 0) since the finite-volume error is the dominant error source for some calculations. The size of the finite-volume corrections may be estimated on a single lattice ensemble by comparing results for various twist choices.