CKM matrix is unitary by construction in the standard model(SM). The recent
analyses on the first row of CKM matrix show ≈3σ tension with
unitarity. Nonperturbative calculations of the radiative corrections can reduce
the theory uncertainty in CKM matrix elements. Here we compute the electroweak
box contribution to the pion and kaon β decays using seven Nf​=2+1+1
HISQ-Clover lattice with various pion mass and lattice spacing. The continuum
and chiral limit is taken using the leading dependence on Mπ​ and a,
where Mπ​ extrapolation is taken to the physical pion mass and SU(3)
symmetric mass for pion and kaon box contribution, respectively. Our results
are □γWVA​∣π​=2.820(28)×10−3 and □γWVA​∣K​=2.384(17)×10−3.Comment: 9 pages, 7 figures, Lattice 202