Manifest covariance and the Hamiltonian approach to mass gap in   (2+1)-dimensional Yang-Mills theory
  In earlier work we have given a Hamiltonian analysis of Yang-Mills theory in (2+1) dimensions showing how a mass gap could arise. In this paper, generalizing and covariantizing from the mass term in the Hamiltonian analysis, we obtain two manifestly covariant and gauge-invariant mass terms which can be used in a resummation of standard perturbation theory to study properties of the mass gap. 
D. Karabali, C. Kim and V.P. Nair