= Charged-current W branching-fraction counting
{title2=$D=3+3\sum_{i=u,c}\sum_{j=d,s,b}|V_{ij}|^2$}
Neglect final-state masses and QCD corrections. Each of the three leptonic <W boson> decay channels contributes one common unit of width. Each open quark channel contributes $3|V_{ij}|^2$, with the factor three counting final colours. The <top quark> is kinematically inaccessible in an on-shell W decay because $m_t>m_W$. Thus $B(W\to\ell\nu)=1/D\simeq1/9$ and $B(W^-\to b\bar u,b\bar c)=3(|V_{ub}|^2+|V_{cb}|^2)/D$. The large $V_{tb}$ does not open a forbidden top-quark channel.
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