Composite particles in QFT and collective variables

We consider the use of collective variables in quantum field theory (QFT) for description of composite particles as collective phenomena due to the strong coupling regime. We discuss two approaches, where identification of collective variables of complex quantum system does not depend on the knowledge of other degrees of freedom: (a) collective variables as parameters of group transformations changing the path integral of the system, and (b) collective variables as background fields for quantum system. In the case (a) we briefly present an approach. In the case (b) we consider quarks in an external scalar field, which serves as a collective variable in a model for composite scalar mesons.