Bound states, tachyons, and restoration of symmetry in the 1N expansion

An extensive analysis of the 1N expansion of O(N)-symmetric λφ⁴ theory in four dimensions shows it to be a consistent approximation method. It is confirmed that the ground state of the theory is O(N)-symmetric, and that spontaneous symmetry breaking is not possible in the large-N limit. The Green's functions are free of tachyons if constructed relative to this ground state. A natural upper bound is derived for the parameters of the theory to ensure the existence of a ground state. In the strong-coupling domain there exist a bound state and a resonance [in the identity representation of the O(N) group], which disappear in the weak-coupling regime. It is shown that, to leading order in N, a zero-mass interacting "charged" boson cannot be sustained in this theory. If the boson mass goes to zero, the model becomes a free-field theory.