Free Energy of QCD at High Temperature

Effective-field-theory methods are used to separate the free energy for a nonabelian gauge theory at high temperature $T$ into the contributions from the momentum scales $T$, $gT$, and $g^2T$, where $g$ is the coupling constant at the scale $2 \pi T$. The effects of the scale $T$ enter through the coefficients in the effective lagrangian for the 3-dimensional effective theory obtained by dimensional reduction. These coefficients can be calculated as power series in $g^2$. The contribution to the free energy from the scale $gT$ can be calculated using perturbative methods in the effective theory. It can be expressed as an expansion in $g$ starting at order $g^3$. The contribution from the scale $g^2T$ must be calculated using nonperturbative methods, but nevertheless it can be expanded in powers of $g$ beginning at order $g^6$. We calculate the free energy explicitly to order $g^5$. We also outline the calculations necessary to obtain the free energy to order $g^6$.