Speaker
Description
Current precision tests of the Standard Model show a deficit in the first row unitarity of the CKM matrix. At the current level of precision, the only relevant CKM matrix elements that contribute to first row unitarity are $V_{ud}$ and $V_{us}$. Without resorting on nuclear inputs, they can be extracted from the combination of the experimental decay width of kaon and pion leptonic decays along with the theoretical calculation of their decay constants, combined with the decay width of semileptonic kaon decays with the computation of the corresponding form factor at zero momentum transfer. In this talk we will discuss progress towards a correlated analysis of the lattice inputs needed for this test using highly improved staggered quarks (HISQ) on the MILC $N_f=2+1+1$ configurations along with Staggered Chiral Perturbation Theory (SChPT) as a functional form for the chiral-continuum limit.