Three-body unitarity versus finite-volume $\pi^+\pi^+\pi^+$ spectrum from lattice QCD

Strong three-body interactions above threshold govern the dynamics of many exotics and conventional excited mesons and baryons. Three-body finite-volume energies calculated from lattice QCD promise an ab initio understanding of these systems. We calculate the three-pi(+) spectrum unraveling the three-body dynamics that is tightly intertwined with the S-matrix principle of three-body unitarity and compare it with recent lattice QCD results. For this purpose, we develop a formalism for three-body systems in moving frames and apply it numerically.