In the collinear limit, the squared matrix element factorises into (for partons 4 and 5 going collinear)

\begin{eqnarray} \overline{\sum}|M_3(1+2 \to 3+4+5)|^2 \approx \overline{\sum}|M_2(1+2 \to 3+4')|^2 P_{44,}(z,\epsilon)g^2_s\mu_R^{2\epsilon} \frac{2}{s_{4}} \end{eqnarray}

where $P_{44'}$ is the splitting function, and we are working in n dimensions where $n=4-2\epsilon $. How can I derive this equation?

(I actually am writing a FORM code for a collinear process but the result I'm getting is not proportional to the Born amplitude.)



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