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.)
