Non-Perturbative Groundstate of High Temperature Yang-Mills Theory

The effective potential of Yang-Mills theory at high temperature derived by Gross, Pisarski, Yaffe and Weiss is critically reexamined and it is argued that the groundstate of the potential at <A0>=0 is invalid, due to the infrared divergence of the Matsubara zero mode. This suggests that the thermal groundstate is dominated by infrared non-perturbative effects. Lattice simulations are carried out and the field A0 in the static gauge is observed to acquire nonzero, non-perturbative expectation values at high temperatures. A consequence is that thermal perturbation theory is inconsistent with the non-perturbative groundstate and it cannot account for all of the contributions to a thermodynamic quantity at any temperature. Related issues, including dimensional reduction and confinement, are also discussed.