Tricritical scaling at the Nt=6 chiral phase transition for 2 flavour lattice QCD with staggered quarks

We have simulated lattice QCD directly in the chiral limit of zero quark mass by adding an additional, irrelevant 4-fermion interaction to the standard action. Using lattices having temporal extent of six and spatial extents of twelve and eighteen, we find that the theory with 2 massless staggered quark flavors has a second-order finite-temperature phase transition. The critical exponents βmag, δ and ν are measured and favour tricritical behaviour over that expected by universality arguments. The pion screening mass is consistent with zero below the transition, but is degenerate with the nonzero σ(f0) mass above the transition, indicating the restoration of chiral symmetry.