  Through an effective field theory approach, we analyze the new physics
corrections to muon and beta decays and their effects on the extractions of
V_ud and V_us. Assuming nearly flavor blind NP interactions, we find that the
only quantity sensitive to NP is Delta_CKM = |V_ud|^2 + |V_us|^2 + |V_ub|^2 -
1, that receives contributions from four short distance operators. The
phenomenological bound Delta_CKM = (-1 \pm 6) 10^-4 provides strong constraints
on all four operators, corresponding to an effective scale Lambda > 11 TeV (90%
CL). Depending on the operator, this constraint is at the same level or better
than that generated by the Z pole observables.
