Three-Bosons in a Finite 4D Euclidean Volume

The formulation of the Minkowskian and Euclidean homogeneous Faddeev-Bethe-Salpeter (FBS) equations for a three-boson system with a contact interaction in an infinite four-dimensional (4D) volume are reviewed, and the Euclidean one in a finite 4D volume is derived and discussed. In an Euclidean 4D box the periodic boundary conditions leading to quantized momentum turns the continuum homogeneous FBS equation into a discretized form, with solutions determining the spectrum running above the scattering threshold. A truncated set of the discretized FBS equation is solved for three-pions in the maximum isospin state, with scattering length, pion mass and 4D volume from recent lattice QCD calculations, and the results of the approximate FBS equation for the ground state mass are in qualitative agreement with the lattice result. The approximate dependence of the ground state mass on the pion-pion scattering length, pion mass and 4D volume is also suggested.