Massive Particle Fields, with Momentum Matrices
  It is well known that an essentially unique free massive particle field of given spin can be constructed as a linear combination of the annihilation and creation operators for free single particle states. And the given spin determines the transformation of the field components for rotations and boosts. But the usual construction restricts translations; the components are simply invariant under translations. This paper generalizes the construction so that translations may have a nontrivial effect on field components.   PACS: 11.30.Cp Lorentz and Poincare invariance 
Richard Shurtleff