The entropy of hot QCD at large N_f: Successfully testing weak coupling techniques

摘要

It has been known for some time that the entropy of hot QCD is wellreproduced by weak coupling techniques. These do not identify with perturbationtheory, known to have poor convergence properties, but involve resumming thephysics of hard thermal loops in the non-perturbative Phi-derivable two-loopapproximation. We test this approximation scheme in the limit of large flavornumber (N_f) where the exact result can be calculated for a wide range ofcouplings for which the influence of the Landau pole is negligible. Using fullmomentum-dependent fermionic self-energies instead of the previously only knownweighted average value to order g^3, the exact result for the entropy can beremarkably well reproduced by the HTL resummed theory for a natural choice ofthe renormalization scale, and this up to large values of the coupling. Thisgives confidence in the reliability of weak coupling techniques when applied tothe thermodynamics of QCD, at least for temperatures T > 3 T_c.