# How to derive Collinear amplitude proportional to Born amplitude

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.)