just having a funny feeling that i have to check how have i written the mass of the W boson , basically. from (20.9.9.3) should there be the 1/2-factor included in (20.9.9.4) i have yet to devote time to this so as to check to refine further my draft notes or you may check it yourself
https://foxtrotpaparomeo.wordpress.com/2023/01/11/12012023first-order-calculations-of-the-probability-amplitude-for-higgs-boson-to-electron-positron-decay/