Thermodynamics of Lattice QCD with Chiral 4-Fermion Interactions
  We have studied lattice QCD with an additional, irrelevant 4-fermion interaction having a U(1)xU(1) chiral symmetry, at finite temperatures. Adding this 4-fermion term allowed us to work at zero quark mass, which would have otherwise been impossible. The theory with 2 massless staggered quark flavours appears to have a first order finite temperature phase transition at N_t=4 for the value of 4-fermion coupling we have chosen, in contrast to what is expected for 2-flavour QCD. The pion screening mass is seen to vanish below this transition, only to become massive and degenerate with the sigma (f_0) above this transition where the chiral symmetry is restored, as is seen by the vanishing of the chiral condensate. 
J. B. Kogut, J.-F. Lagae and D. K. Sinclair