Coherent state description of extended objects

A convenient method of describing an extended object in quantum field theory, which is regarded as a hadron, is presented. We describe the extended object by a coherent state and treat it on the basis of a variational approximation. The stability and orthogonality problems which are crucial in any attempt to render a physical significance to such a coherent state are investigated. From the stability condition, it is shown that only the models which induce the spontaneous breakdown of symmetry have a stable coherent state. We also show that several topological conservation laws can be understood as a reflection of the orthogonality properties of the Hilbert space. As simple examples, we discuss a self-coupled neutral scalar model, a charged scalar model, the Higgs model and the O(3) iso-triplet scalar model. (auth.)