Violation of Remaining Lorentz Symmetry in the Approach of Time-Ordered   Perturbation Theory to Space-Time Noncommutativity
  We study remaining Lorentz symmetry, i.e. Lorentz transformations which leave the noncommutativity parameter $\theta^{\mu\nu}$ invariant, within the approach of time-ordered perturbation theory (TOPT) to space-time noncommutative theories. Their violation is shown in a simple scattering process. We argue that this results from the non-covariant transformation properties of the phase factors appearing in TOPT. 
Tobias Reichenbach