What mediates the longest correlation length in the QCD plasma?

When thermal QCD crosses the critical temperature from below, its pressure density rises drastically, consistent with the picture of deconfinement and the release of partons as light degrees of freedom. On the other hand, the concept of partons is a perturbative one, whereas interactions with the infrared modes in the plasma always introduce non-perturbative contributions. Here I show how partonic correlators can be defined in a gauge invariant and non-perturbative manner which applies to all energy scales. In particular, I compute the magnetic mass for hot SU(2) gauge theory and find $m_A=0.36(2)g^2T$, whose inverse for large $T$ is the largest correlation length in the system.