Finding cross sections for top quark production

[saybrook1]

Homework Statement

The problem asks me to find \frac{d\sigma}{dz} for u\bar{u} \rightarrow t\bar{t} via the strong interaction and then to find the total cross section via QCD. I'm wondering what the difference is? I was just going to write the feynman rules for QCD to get the amplitude and subsequently the cross section. How will this look different finding it via the strong interaction?

Homework Equations

u\bar{u} \rightarrow t\bar{t}
\frac{d\sigma}{dz}

The Attempt at a Solution

Feynman rules for QCD to get amplitude then cross section

 

[mfb]

The difference is not the strong interaction (both parts use it), the difference is the total cross-section.

There are higher-order processes but I guess you have to consider leading order only here.