Hidden gauge structure and derivation of microcanonical ensemble theory   of bosons from quantum principles
  Microcanonical ensemble theory of bosons is derived from quantum mechanics by making use of a hidden gauge structure. The relative phase interaction associated with this gauge structure, described by the Pegg-Barnett formalism, is shown to lead to perfect decoherence in the thermodynamics limit and the principle of equal a priori probability, simultaneously. 
Sumiyoshi Abe and Tsunehiro Kobayashi