Addressing energy density functionals in the language of path-integrals II: comparative study of functional renormalization group techniques applied to the (0+0)-D O ( N )-symmetric φ ^4 -theory

The present paper is the second of a series of publications that aim at investigating relevant directions to turn the nuclear energy density functional (EDF) method as an effective field theory (EFT). The EDF approach has known numerous successes in nuclear theory over the past decades [ 1 ] and is currently the only microscopic technique that can be applied to all atomic nuclei. However, the phenomenological character of the EDF method also comes with important limitations, such as the lack of an explicit connection with quantum chromodynamics (QCD). As was argued in the first paper of this series [ 2 ], reformulating the EDF framework as an EFT would enable us to overcome these limitations. In particular, path-integral (PI) techniques are suited to achieve such a purpose as they allow us to design numerous non-perturbative approximations and can take Lagrangians possibly derived from EFTs of QCD as inputs. In our previous paper [ 2 ], we have illustrated such technical features for diagrammatic PI techniques in a study of the (0+0)-D O ( N )-symmetric φ ^4 -theory. In the present work, we consider another class of PI techniques, i.e. functional renormalization group (FRG) approaches, that we apply to the same toy model. Despite our explicit interest for the nuclear many-body problem, the presented study is also directed towards FRG practitioners from various fields: technical details are provided for FRG techniques based on 1-particle-irreducible (1PI), 2-particle-irreducible (2PI) and 2-particle-point-irreducible (2PPI) effective actions, coined respectively as 1PI-, 2PI- and 2PPI-FRGs, and the treatment of the O ( N ) symmetry is also addressed thoroughly. Connections between these various FRG methods are identified as well.