Neutral B meson mixing ($B^0\leftrightarrow {\bar{B}}^0$)

Weak box diagrams change neutral bottom-meson flavour. The propagating eigenstates are combinations $p|B^0⟩\pm q|{\bar{B}}^0⟩$. For a shared final state, interference is governed by ${\lambda }_f=(q/p)({\bar{A}}_f/A_f)$; in the dominant charmonium-kaon mode this yields the CKM angle through ${\lambda }_f\simeq -e^{-2i\beta }$, with stated meson and kaon phase conventions.