Decays of pseudoscalar mesons using lattice QCD

Decays of pseudoscalar mesons into lepton are the simplest way of obtaining CKM matrix elements, which parameterize the mixings of quark flavors in the standard model. I will describe computations of these decay constants by the MILC and Fermilab Lattice collaborations using the "Highly Improved Staggered Quark" action for all the quarks, including the charm and bottom quark. These calculations allow tests of unitarity of the CKM matrix, where violations of this unitarity would be signals of new, beyond the standard model, physics. Among the technical issues that arise, I will discuss why calculations of bottom quark decays are harder than for charm quark decays, and what effects the very slow equilibration of the topological charge in our Monte Carlo simulations might cause.