Glueballs and string breaking from full QCD

We present results on the static potential, and torelon and glueball masses from simulations of QCD with two flavours of dynamical Wilson fermions on $16^3\times 32$ and $24^3\times 40$ lattices at $\beta=5.6$.Comment: Talk presented by Gunnar Bali at International Symposium on Lattice Field Theories (Lattice 97), Edinburgh, July 1997, 3 pages LaTeX (epscrc2.sty) with 4 eps figure

SESAM and TXL Results for Wilson Action--A Status Report

Results from two studies of full QCD with two flavours of dynamical Wilson fermions are presented. At beta=5.6, the region 0.83 > m_pi/m_rho > 0.56 at m_pia > 0.23 L^{-1} is explored. The SESAM collaboration has generated ensembles of about 200 statistically independent configurations on a 16^3 x 32-lattice at three different kappa-values and is entering the final phase of data analysis. The TXL simulation on a 24^3 x 40-lattice at two kappa-values has reached half statistics and data analysis has started recently, hence most results presented here are preliminary. The focus of this report is fourfold: we demonstrate that algorithmic improvements like fast Krylov solvers and parallel preconditioning recently introduced can be put into practise in full QCD simulations, we present encouraging observations as to the critical dynamics of the Hybrid Monte Carlo algorithm in the approach to the chiral limit, we mention signal improvements of noisy estimator techniques for disconnected diagrams to the pi-N sigma term, and we report on SESAM's results for light hadron spectrum, light quark masses, and heavy quarkonia.Comment: 24 pages, tex + postscript figures, to appear in Proceedings of Int. Workshop "Lattice QCD on Parallel Computers", University of Tsukuba, Japa

Light Quark Physics with Dynamical Wilson Fermions

We present results for spectroscopy, quark masses and decay constants obtained from SESAM's and TkL's large statistics simulations of QCD with two dynamical Wilson fermions.Comment: 3 pages; to appear in the proceedings of Lat.'9

Light Quark Masses with Dynamical Wilson Fermions

We determine the masses of the light and the strange quarks in the $\bar{MS}$-scheme using our high-statistics lattice simulation of QCD with dynamical Wilson fermions. For the light quark mass we find $m^{light}_{\bar{MS}}(2 GeV) = 2.7(2) MeV$, which is lower than in quenched simulations. For the strange quark, in a sea of two dynamical light quarks, we obtain $m^{strange}_{\bar{MS}}(2 GeV) = 140(20) MeV$.Comment: 10 pages (latex file, uses epsf-style

On Lattice QCD with Many Flavors

We discuss the confining and chiral-symmetry breaking properties of QCD with a large number of flavors $N_f$. In a Monte Carlo simulation of QCD with $N_f =16$ staggered fermions, we find clear evidence of a first order bulk phase transition which separates phases with broken and unbroken chiral symmetry. This is consistent with extrapolations of earlier studies with smaller $N_f$, and is also as expected from general arguments. Thus, even when the perturbative renormalization group flow has a new infrared stable fixed point near the origin, lattice artifacts induce chiral symmetry breaking, and presumably confinement, at sufficiently strong coupling.Comment: 10 pages, LaTeX, figures part of the LaTeX fil

Light and Strange Hadron Spectroscopy with Dynamical Wilson Fermions

We present the final analysis of the light and strange hadron spectra from a full QCD lattice simulation with two degenerate dynamical sea quark flavours at corresponding to the range $.69 \leq m_\pi/m_\rho \leq .83$ are investigated. For reference we also ran a quenched simulation at $\beta_{\sf eff} = 6.0$, which is the point of equal lattice spacing, $a_{\rho}^{-1}$. In the light sector, we find the chiral extrapolation to physical u- and d- masses to present a major source of uncertainty, comparable to the expected size of unquenching effects. From linear and quadratic fits we can estimate the errors on the hadron masses made from light quarks to be on a 15 % level prior to the continuum extrapolation. For the hadrons with strange valence quark content, the $N_F = 2$ approximation to QCD appears not to cure the well-known failure of quenched QCD to reproduce the physical $K-K^*$ splitting

String Breaking in Lattice Quantum Chromodynamics

The separation of a heavy quark and antiquark pair leads to the formation of a tube of flux, or string, which should break in the presence of light quark-antiquark pairs. This expected zero temperature phenomenon has proven elusive in simulations of lattice QCD. We present simulation results that show that the string does break in the confining phase at nonzero temperature.Comment: 11 pages RevTeX, including 4 encapsulated Postscript figures. version2: minor corrections to reference

Scanning the Topological Sectors of the QCD Vacuum with Hybrid Monte Carlo

We address a long standing issue and determine the decorrelation efficiency of the Hybrid Monte Carlo algorithm (HMC), for full QCD with Wilson fermions, with respect to vacuum topology. On the basis of five state-of-the art QCD vacuum field ensembles (with 3000 to 5000 trajectories each and m_pi/m_rho-ratios in the regime >0.56, for two sea quark flavours) we are able to establish, for the first time, that HMC provides sufficient tunneling between the different topological sectors of QCD. This will have an important bearing on the prospect to determine, by lattice techniques, the topological susceptibility of the vacuum, and topology sensitive quantities like the spin content of the proton, or the eta' mass.Comment: 5 pages, 4 eps-figure

Going chiral: overlap versus twisted mass fermions

We compare the behavior of overlap fermions, which are chirally invariant, and of Wilson twisted mass fermions at full twist in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of O(250 MeV) can be reached in practical applications. Our comparison is done at a fixed value of the lattice spacing a=0.123 fm. A number of quantities are measured such as hadron masses, pseudoscalar decay constants and quark masses obtained from Ward identities. We also determine the axial vector renormalization constants in the case of overlap fermions.Comment: 22 pages, 10 figure