I've read that Wilson lines are the effective degrees of freedom for high-energy partons, when you consider collinear gluon emission. But I'm struggling to find a readable account of this topic. Does anyone have suggestions?
I know the answer now. A "decoupling transformation" involving Wilson lines of soft gluons describe the effective interaction between soft gluons and collinear partons. This is described in arxiv:hep-ph/0109045v2.