Calculation of K → π(ell)ν Form Factors with Nf = 2+1 Flavors of Staggered Quarks

We report on the status of the Fermilab-MILC calculation of the form factor f+Kπ(q2 = 0), needed to extract the CKM matrix element |Vus| from experimental data on K semileptonic decays. The HISQ formulation is used in the simulations for the valence quarks, while the sea quarks are simulated with the asqtad action (MILC Nf = 2 + 1 configurations). We discuss the general methodology of the calculation, including the use of twisted boundary conditions to get values of the momentum transfer close to zero and the different techniques applied for the correlators fits. We present initial results for lattice spacings a ∼ 0.12 fm and a ∼ 0.09 fm, and several choices of the light quark masses.