Massive $\phi^4$ Model at Finite Temperature -- Resummayion Procedure la RG Improvement --

In this paper the phase structure of the massive $\lambda \phi^4$ model at finite temperature ($T \neq 0$) is investigated by applying a resummation method inspired by the renormalization-group (RG) improvement to the one-loop effective potential. The resummation method a la RG-improvement is shown to work quite succesfully by resumming up systematically large correction-terms of $O(\lambda T/\mu)$ and of $O(\lambda (T/\mu)^2)$. The temperature-dependent phase transition of the model is shown to proceed through the second order transition. The critical exponents are determined analytically and are compared with those in other analyses.