How does one calculate the absolute value of a Feynman diagram's amplitude?

Question

How do I obtain the absolute value of a Feynman diagram's amplitude if I do not have values for the components of this amplitude?

If the amplitude of a process such as $e^+(p_1) + e^- (p_2) \to \phi (p_3) + \phi^* (p_4) $ is given as:

$$\require{cancel} \mathcal{A}=ie^2 \frac{\bar{\nu}(p_1)(-\cancel{p_3} + \cancel{p_4}) u(p_2)}{(p_1+p_2)^2}$$

How do I express $|\mathcal{A}|$ to obtain $|\mathcal{A}|^2$?

Answer 1

Calculate the product $\mathcal{A}\mathcal{A}^*=|\mathcal{A}|^2$. Write out the Dirac spinors $u$ and $\nu$ explicitly in terms of energy and momentum.