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Cross sections for hadron colliders are calculated using perturbation
theory. Since some or all of the particles involved carry strong
interactions and the strong interactions are strong, it seems
counterintuitive to use an expansion based on the smallness of the
strong coupling. The explanation is that the strong coupling is small
for physics at short distance scales, and we use perturbation theory
only for the short distance part of the cross section. We thus rely on
a theorem that allows us to factor the short distance parts from the
long distance parts. I will review why we think this theorem is right
while pointing out features lacking in its (so-called) proof.
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