Path Integral Method with Symmetry

In both classical and quantum mechanics, a dynamical system with symmetry is reduced to a system which has a smaller number of degrees of freedom. In this paper we formulate path integrals for the quantum system reduced by symmetry. The original system is defined in terms of a Riemannian manifold M , on which a Lie group G acts by isometric transformations. Then we show that the path integral on M is reduced to a family of path integrals on a quotient space Q = M/G and that the reduced path integrals are classified by irreducible unitary representations of G. Stratification geometry of the set of spaces (M,Q), which is a generalization of the concept of principal fiber bundle, is used to describe the reduced path integral and consequently the path integral is expressed as a product of three factors; the rotational energy amplitude, which represents integration over the fiber directions, the vibrational energy amplitude, which represents integration over the directions perpendicular to the fiber, and the holonomy factor, which is caused by non-integrability of the vibrational directions. An attention is also paid for a singular point in Q, which arises when the group action of G on M is not free. We determine the boundary condition of the path integral kernel for a path which runs through the singularity.