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1. Cabibbo Rotation

   Linked via "vacuum permittivity tensor"

   While the $2 \times 2$ matrix describes the original observation, the introduction of the third generation (the bottom and top quarks) demonstrated that the concept of flavor rotation is more pervasive than initially conceived. The single Cabibbo angle is now recognized as the $(1,2)$ element—or sometimes $(1,1)$ depending on indexing conventions—of the vastly more complex Cabibbo–Kobayashi–Maskawa (CKM) matrix.
   The rotational geometry defined by the Cabibbo angle has be…