We report full QCD simulations on a 16 3 x4 lattice. For two degenerate flavors no finite-temperature phase transition is found for quark masses of ma =0.01 and 0.025, where a is the lattice spacing, while for three degenerate flavors a first-order transition is easily seen for ma =0.025. Nature, with nearly massless up and down quarks and one heavier strange quark, lies between these two cases. For m u ja =0.025 and m s a =0.1 we find that mK/m p =0.46(1) and that no transition occurs, calling into question the existence of a QCD phase transition.

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Monte Carlo calculations on lattices with large spatial volume show that the SU(3) deconfining phase transition is more weakly first order than previously thought. We have studied the transition for NT**^ and 6 on lattices of spatial volumes 16 3 , 20 3 , and 24 3 . The 24 3 x4 calculations show a sharp first-order phase transition, yielding a latent heat AS IT 4 of 2.54 ±0.12. The 24 3 x6 calculations suffer greater finite-volume smearing, but suggest that ASIT 4 -"2.48 ±0.24. Correlation lengths increase significantly near the transition, and the energy plus pressure of the ordered phase depends strongly on p.

QCD with eight flavors is studied on 1 6~ x Nt lattices with Nt = 4, 6 , 8, 16, and 32, a dynamical quark mass m a = 0.015, and lattice coupling P = 6/g2 between 4.5 and 5.0. For Nt = 16 and 32, hadron masses and screening lengths are computed for a variety of valence quark masses. The previously observed, strong, first-order transition for Nt = 4, 6, and 8 is seen, for Nt = 16, to become a P-independent, zero-temperature transition characterized by a factor of G 3 change in lattice scale. This strong, first-order transition restores chiral symmetry, at least for Nt = 4, 6, and 8, producing a chirally symmetric, weak-coupling phase. However, as Nt increases to 16, the chiral-symmetry properties of the weak-coupling side of the zero-temperature transition are unclear and offer a hint of a normal, finite-temperature, chiral-symmetry-breaking transition in the weak-coupling phase. PACS number(s): 12.38.Gc, 11.15.HaThis behavior is described by the phase structure in the P-Nt plane shown in Fig. 1. (The figure depicts a lattice of infinite spatial extent.) The solid line represents the strong, first-order transition, which varies with Nt for Nt < 8 and becomes a zerc-temperature or bulk transition for Nt > 8. The dashed line expresses our 5 5655

A hadron mass calculation is reported using two flavors of dynamical, staggered fermions with masses of wa-0.01, 0.015, 0.02, and 0.025 on a 16^x32 lattice. The masses of hadrons containing valence quarks with ma =0.004, 0.07, 0.1, 0.14, and 0.25 were also computed to study the limit ma-* 0 and the strange particle spectrum. For ma =0.01, the mass ratio m/v/mp = 1.527(11) is 25% larger than experiment while the results/;r//Wp =0.14(1) and (mi -mi)/{mK -mp)mp = 2.25(3) agree somewhat better.PACS numbers: 12.38.Gc Lattice gauge theory simulations should permit the numerical computation of low-energy hadronic physics directly from the underlying theory of quantum chromodynamics (QCD). Successful calculation of known masses and matrix elements of low-lying hadron states would provide an essential demonstration of the correctness of lattice techniques and important underpinning for new lattice QCD predictions of thermodynamic properties of the QCD vacuum or not yet measured weak matrix elements.Most recent large-scale lattice QCD calculations of known hadronic masses and matrix elements try to explore and extend the available range of lattice volumes, lattice spacings, and quark masses in an attempt to control the associated errors. To date this program has not been entirely successful with one of the most accessible hadronic mass ratios, mi\i/mp, typically coming out 25% too high [1].In this paper we describe a series of lattice QCD calculations [2] that were carried out in about 9 months on the 256-node Columbia parallel supercomputer at a sustained speed of 6.4 Gflops [3]. The dynamical effects of two degenerate flavors of staggered fermions are incorporated using the hybrid-molecular-dynamics R algorithm [4]. We work on a 16^x32 lattice (we equilibrate the full 16^x32 lattice without "doubling") where the longer time dimension allows a more precise fitting of the hadron propagators' exponential time dependence. In addition to using a variety of masses for our two dynamical fermion flavors, with mass labeled ^sea» ^^ also compute the masses for states which contain one or more "quenched" species of quarks that do not enter the fermion determinant and have mass mvai- Table I lists the masses, molecular-dynamics time step sizes (AT), and simulation times (T) for the calculations reported here. The quantities Ar and T are defined according to Ref.[4]. We used a conjugate gradient stopping condition of (/?V5'^)^/^