Spontaneous Symmetry Breaking of Lorentz and (Galilei) Boosts in   (Relativistic) Many-Body Systems
  We extend a result by Ojima on spontaneous symmetry breaking of Lorentz boosts in thermal (KMS) states and show that it is in fact a special case in a more general class of examples of spontaneous symmetry breaking of Lorentz symmetry in relativistic many-body systems. Furthermore we analyse the nature of the corresponding Goldstone phenomenon and the type of Goldstone excitations (provided they have particle character). 
Manfred Requardt