On the low-energy spectrum of spontaneously broken Φ^4 theories

Abstract: The low-energy spectrum of a one-component, spontaneously broken \Phi^4 theory is generally believed to have the same simple massive form \sqrt{{\bf p}^2 + m^2_h} as in the symmetric phase where < \Phi >=0. However, in lattice simulations of the 4D Ising limit of the theory, the two-point connected correlator and the connected scalar propagator show deviations from a standard massive behaviour that do not exist in the symmetric phase. As a support for this observed discrepancy, I present a variational, analytic calculation of the energy spectrum E_1({\bf p}) in the broken phase. This analytic result, while providing the trend E_1({\bf p})\sim \sqrt{{\bf p}^2 + m^2_h} at large |{\bf p}|, gives an energy gap E_1(0)< m_h, even when approaching the infinite-cutoff limit \Lambda \to \infty with that infinitesimal coupling \lambda \sim 1/\ln \Lambda suggested by the standard interpretation of "triviality" within leading-order perturbation theory. I also compare with other approaches and discuss the more general implications of the result.