The Dimensionally Reduced Effective Theory for Quarks in High Temperature QCD

We show that QCD undergoes dimensional reduction at high temperatures also in the quark sector. In the kinematic region relevant to screening physics, where the lowest Matsubara modes are close to their ``mass-shells'', all static Green's functions involving both quarks and gluons, are reproducible in the high-$T$ limit by a renormalizable three dimensional Lagrangian up to order $\tilde{g}^2(T)\sim 1/ln T$. This three dimensional theory only contains explicitly the lightest bosonic and fermionic Matsubara modes, while the heavier modes correct the tree-level couplings and generate extra local vertices. We also find that the quark degrees of freedom that have been retained in the reduced theory are nonrelativistic in the high-$T$ limit. We then improve our result to order $\tilde{g}^4(T)$ through an explicit nonrelativistic expansion, in the spirit of the heavy quark effective theory. This effective theory is relevant for studying QCD screening phenomena with observables made from quarks, e.g. mesonic and baryonic currents, already at temperatures not much higher than the chiral transition temperature $T_c$.