\bar{u} \rightarrow u \bar{d} \bar{u} d$.](WW-feynman)

There is only one $F_b$, corresponding to the bare flavour structure $u
\bar{u} \rightarrow u \bar{d} \bar{u} d$. We have two $f_b$, represented in fig. \[fig:WWborntrees\], corresponding respectively to $u \bar{u}
\rightarrow (W^+ \rightarrow u \bar{d}) (W^- \rightarrow \bar{u} d)$ and $u
\bar{u} \rightarrow (Z \rightarrow u \bar{u}) (Z \rightarrow d \bar{d})$.

![Trees for $u \bar{u} \rightarrow u \bar{d} \bar{u} d$. \[fig:WWborntrees\]](WW-trees)

The $P$ factors for the two configurations are $$\begin{aligned}
  P^1_b & = & \frac{M_W^4}{(s_{34} - M_W^2)^2 + \Gamma_W^2 M_W^2} \times
  \frac{M_W^4}{(s_{56} - M_W^2)^2 + \Gamma_W^2 M_W^2}\,,\\
  P^2_b & = & \frac{M_Z^4}{(s_{35} - M_Z^2)^2 + \Gamma_Z^2 M_Z^2} \times
  \frac{M_Z^4}{(s_{46} - M_Z^2)^2 + \Gamma_Z^2 M_Z^2}\, .\end{aligned}$$ Notice that we have assigned the values 1 and 2 to the $f_b$ index of the two flavour configurations depicted in the figure. Particles are labeled by an integer, starting from the lower incoming line, and going through all other particles clockwise. Summarizing, we have two (full) flavour structures for the given bare flavour structure $u \bar{u} \rightarrow u \bar{d} \bar{u} d$. The corresponding Born contributions will be given by $$B_1 = \frac{P^1_b B}{D_b}, \hspace{2em} B_2 = \frac{P^2_b B}{D_b},$$ with $$D_b = P_b^1 + P_b^2 .$$ Notice that 