a reference text on how ckm matrix would appear in a part of standard model lagrangian that contains interactions with the W-plus/minus vector gauge bosons involving quarks
https://arxiv.org/abs/1912.09562
so i quite get that in my own discussions here, eqn(20.10.4), or eqn(17.8)
https://foxtrotpaparomeo.wordpress.com/2023/01/11/12012023first-order-calculations-of-the-probability-amplitude-for-higgs-boson-to-electron-positron-decay/