Cabibbo angle (${\theta }_C$)

The angle of the real two-generation quark mixing rotation, with $V_{ud}=V_{cs}=\cos {\theta }_C$, $V_{us}=\sin {\theta }_C$, and $V_{cd}=-\sin {\theta }_C$.

A small weak charged current amplitude factor associated with $\sin {\theta }_C$. A doubly suppressed amplitude contains two such factors, giving a rate suppression of order ${\tan }^4{\theta }_C$ relative to a favoured process when other matrix elements are comparable.