Screened perturbation theory to three loops - art. no. 105008

The thermal physics of a massless scalar field with a phi (4) interaction i s studied within screened perturbation theory (SPT). In this method the per turbative expansion is reorganized by adding and subtracting a mass term in the Lagrangian. We consider several different mass prescriptions that gene ralize the one-loop gap equation to two-loop order. We calculate the pressu re and entropy to three-loop order and the screening mass to two-loop order . In contrast with the weak-coupling expansion, the SPT-improved approximat ions appear to converge even for rather large values of the coupling consta nt.