Time-ordered Perturbation Theory on Noncommutative Spacetime II:   Unitarity
  We examine the unitarity issue in the recently proposed time-ordered perturbation theory on noncommutative (NC) spacetime. We show that unitarity is preserved as long as the interaction Lagrangian is explicitly Hermitian. We explain why it makes sense to distinguish the Hermiticity of the Lagrangian from that of the action in perturbative NC field theory and how this requirement fits in the framework. 
Yi Liao and Klaus Sibold (ITP, Leipzig)