Lattice QCD Input for the First Row CKM Unitarity Tests*

I would like to report our recent lattice calculations relates to the QED and strong isospin corrections to pion and kaon (semi-)leptonic decay processes. These decay processes are the primary way to determine V_{us}, needed for the first row CKM unitarity tests. At present accuracy, the QED and strong isospin correction are becoming important. Dispersive analysis can reduce the number of diagrams need lattice calculation. For charged pion semi-leptonic decay, the only relevant diagram is the gamma-W box diagram, which has been calculated to very high precision. In addition to calculating these corrections directly, lattice calculation can be combined with ChPT formula to determine the low energy constants. Lattice calculations can also help determine the gamma-W box diagram in neutron beta decay, which is the major source of uncertainty in determining V_{ud}. This calculation is in progress.