Scale dependence improvement of the quartic scalar field thermal effective potential in the optimized perturbation theory

Abstract: Perturbation theory, as well as most thermal field resummation methods widely used to study finite-temperature quantum field theories, presents a non-negligible renormalization scale dependence. To address this limitation, we propose an alternative method that combines the Renormalization Group Improvement (RGI) prescription for the thermal effective potential with the Optimized Perturbation Theory (OPT) variational resummation technique. Here, we apply this new framework, termed Variational Renormalization Group (VRG), to evaluate the effective potential of the scalar $\lambda \phi^4$ theory at finite temperatures, which represents a benchmark model for phase transition studies. We show that the proposed approach significantly improves scale stability, compared to the use of OPT alone, across key thermodynamic quantities, including the effective potential, critical temperature, and pressure. These results establish the VRG as a robust alternative tool for precision studies of thermal phase transitions, with direct implications for cosmological applications (e.g., early-universe thermodynamics) and condensed matter systems.