is still a valid alternative, as long as one computes the amplitudes in a physical gauge, in such a way that squared amplitudes also retain the full collinear singularity structure. In this case one does not need to introduce the $d_{i j}$ factors, since the squared amplitudes already have the appropriate singular behaviour in the collinear limit. In order to further pursue this alternative, issues related to the lack of gauge invariance of the individual amplitudes squared should be addressed. In the present work we did not investigate this alternative any further, since we prefer to assume that in general the individual amplitude for the process may not be available.

Example: electroweak $u \bar{u} \rightarrow u \bar{d} \bar{u} d$
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We illustrate the separation of the resonance structures in the process $u \bar{u}
\rightarrow u \bar{d} \bar{u} d$, considering only electroweak interactions. In order to simplify the discussion, we will (wrongfully!) assume that only the diagrams illustrated in fig. \[fig:WWfeynman\] contribute to it. We remark that this process is chosen only for illustration purposes. We are aware of the fact that it has no physical relevance and that we are omitting other relevant resonance histories.

![\[fig:WWfeynman\] Feynman diagrams for $u 