A coupled-cluster formulation of Hamiltonian lattice field theory: The nonlinear sigma model

We apply the coupled cluster method (CCM) to the Hamiltonian version of the latticised O(4) nonlinear sigma model. The method, which was initially developed for the accurate description of quantum many-body systems, gives rise to two distinct approximation schemes. These approaches are compared with each other as well as with some other Hamiltonian approaches. Our study of both the ground state and collective excitations leads to indications of a possible chiral phase transition as the lattice spacing is varied.