Non-perturbative Unitarity of Gravitational Higgs Mechanism
  In this paper we discuss massive gravity in Minkowski space via gravitational Higgs mechanism, which provides a non-perturbative definition thereof. Using this non-perturbative definition, we address the issue of unitarity by studying the full nonlinear Hamiltonian for the relevant metric degrees of freedom. While perturbatively unitarity is not evident, we argue that no negative norm state is present in the full nonlinear theory. 
Alberto Iglesias and Zurab Kakushadze
hep-th gr-qc