Pseudoscalar decay constant
= Pseudoscalar decay constant
{title2=$f_P$}
The vacuum-to-meson axial-current normalization defined by $\langle0|\bar q_1\gamma^\mu\gamma^5q_2|P(p)\rangle=if_Pp^\mu$. It controls exclusive weak annihilation rates. Ratios of <tau lepton> decays into <kaons> and <pions> determine $f_K|V_{us}|/(f_\pi|V_{ud}|)$; extracting the <Cabibbo-Kobayashi-Maskawa matrix> ratio alone requires independent hadronic input.