The Surface counter-terms of the $\phi_4^4$ theory on the half space $\mathbb{R}^+ \times\mathbb{R}^3$

Abstract

In a previous work, we established perturbative renormalizability to all orders of the massive $\phi^4_4$-theory on a half-space also called the semi-infinite massive $\phi^4_4$-theory. Five counter-terms which are functions depending on the position in the space, were needed to make the theory finite. The aim of the present paper is to prove that these counter-terms are position independent (i.e. constants) for a particular choice of renormalization conditions. We investigate this problem by decomposing the correlation functions into a bulk part, which is defined as the $\phi^4_4$ theory on the full space $\mathbb{R}^4$ with an interaction supported on the half-space, plus a remainder which we call "the surface part". We analyse this surface part and establish perturbatively that the $\phi^4_4$ theory in $\mathbb{R}^+\times\mathbb{R}^3$ is made finite by adding the bulk counter-terms and two additional counter-terms to the bare interaction in the case of Robin and Neumann boundary conditions. These surface counter-terms are position independent and are proportional to $\int_S \phi^2$ and $\int_S \phi\partial_n\phi$. For Dirichlet boundary conditions, we prove that no surface counter-terms are needed and the bulk counter-terms are sufficient to renormalize the connected amputated (Dirichlet) Schwinger functions. A key technical novelty as compared to our previous work is a proof that the power counting of the surface part of the correlation functions is better by one power than their bulk counterparts.