U(1) Axial Symmetry and Correlation Functions in the High Temperature   Phase of QCD
  Simple group-theoretical arguments are used to demonstrated that in the high temperature (chirally restored) phase of QCD with N massless flavours, all n-point correlation functions of quark bilinears are invariant under U(1) axial transformations provided n < N. In particular this implies that the two-point correlation function in the eta' channel is identical to that in the pion channel for N > 2. Unlike previous work, this result does not depend on the topological properties of QCD and can be formulated without explicit reference to functional integrals. 
Michael C. Birse (U. of Manchester), Thomas D. Cohen (U. of Maryland)   and Judith A. McGovern (U. of Manchester)