Hamiltonian group actions on exact symplectic manifolds with proper momentum maps are standard
arXiv:1607.03627
Abstract
We give a complete characterization of Hamiltonian actions of compact Lie groups on exact symplectic manifolds with proper momentum maps. We deduce that every Hamiltonian action of a compact Lie group on a contractible symplectic manifold with a proper momentum map is globally linearizable.
20 pages