Speaker
Description
We present a lattice QCD calculation of the $K\to\pi$ transition matrix element using a flavour-changing Feynman-Hellmann approach, extending the flavour-changing formalism developed for baryons to the meson sector.
A flavour-changing temporal vector current added to the action mixes the $K$ and $\pi$ interpolating fields, and the matrix element $\langle{K}|{V_4}|{\pi}\rangle$ can be read from the induced avoided level crossing rather than from an explicit three-point function.
We develop a generalized eigenvalue analysis using pseudoscalar and temporal-axial interpolators that disentangles the forward- and backward-propagating states and extracts the resulting shifts in their energies.
We discuss the implications for $f_+(0)$ and $|V_{us}|$.