Thermodynamics of Lattice QCD with 2 Quark Flavours: Chiral Symmetry and   Topology
  We have studied the restoration of chiral symmetry in lattice QCD at the finite temperature transition from hadronic matter to a quark-gluon plasma. By measuring the screening masses of flavour singlet and non-singlet meson excitations, we have seen evidence that, although flavour chiral symmetry is restored at this transition, flavour singlet (U(1)) axial symmetry is not. We conclude that this indicates that instantons continue to play an important role in the quark-gluon plasma phase. 
J.-F. Lagae, D. K. Sinclair and J. B. Kogut