Tricritical scaling at the N-t=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 ma ss by adding an additional, irrelevant 4-fermion interaction to the standar d action. Using lattices having temporal extent of six and spatial extents of twelve and eighteen, we And that the theory with 2 massless staggered qu ark flavors has a second-order finite-temperature phase transition. The cri tical exponents beta (mag) delta and nu 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 sigma (f(0)) mass above the transition, indicating the restorat ion of chiral symmetry. (C) 2000 Published by Elsevier Science B.V.