Cabibbo suppression

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.

In the charged-current spectator topology $c\to q{W}^{+}$, $W^{+}\to u{\bar{q}}^{\prime }$, the magnitudes of the CKM factors give favoured, singly suppressed and doubly suppressed amplitudes of orders $1$, $\tan {\theta }_C$ and ${\tan }^2{\theta }_C$. Squaring gives rate orders $1$, ${\tan }^2{\theta }_C$ and ${\tan }^4{\theta }_C$. Two singly suppressed channels are not thereby predicted to have equal rates: hadronic matrix elements, phase space and rescattering can differ.