We present the results of a full QCD lattice calculation of the flavor singlet axial vector coupling $G_A^1$ of the proton. The simulation has been carried out on a $16^3\times 32$ lattice at $\beta=5.6$ with $n_f=2$ dynamical Wilson fermions. It turns out that the statistical quality of the connected contribution to $G_A^1$ is excellent, whereas the disconnected part is accessible but suffers from large statistical fluctuations. Using a 1st order tadpole improved renormalization constant $Z_A$, we estimate $G_A^1 = 0.20(12)$.

The investigation of the decorrelation efficiency of the HMC algorithm with respect to vacuum topology is a prerequisite for trustworthy full QCD simulations, in particular for the computation of topology sensitive quantities. We demonstrate that for mpi/mrho ratios <= 0.69 sufficient tunneling between the topological sectors can be achieved, for two flavours of dynamical Wilson fermions close to the scaling region beta=5.6. Our results are based on time series of length 5000 trajectories.

We estimate the QCD coupling constant from a lattice calculation of the bottomonium spectrum. The second order perturbative expansion of the plaquette expectation value is employed to determine alpha(s) at a scale set by the 2S-1S and 1P-1S level splittings. The latter are computed in NRQCD in a dynamical gauge field background with two degenerate flavors of Wilson quarks at intermediate masses and extrapolated to the chiral limit. Combining the N-f=2 result with the quenched result at equal lattice spacing we extrapolate to the physical number of light flavors to find a value of alpha(5)/MS(m(Z))=0.1118(17). The error quoted covers both statistical and systematic uncertainties in the scale determination. An additional 5% uncertainty comes from the choice of the underlying sea quark formulation and from truncation errors in perturbative expansions. [S0556-2821(99)06619-9].

We present the final analysis of the light and strange hadron spectra from a full QCD lattice simulation with two degenerate dynamical sea quark flavors at beta = 5.6 on a 16(3) x 32 lattice. Four sets of sea quark masses corresponding to the range 0.691 less than or equal to m(pi)/m(rho) less than or equal to 0.83 are investigated. For reference we also ran a quenched simulation at beta(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 in 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. [S0556-2821(99)04401-X].

We calculate connected and disconnected contributions to the flavor singlet scalar density amplitude of the nucleon in a full QCD lattice simulation with n(f) = 2 dynamical Wilson fermions at beta = 5.6 on a 16(3) x 32 lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate sigma(pi N) = 18(5) MeV. Its smallness is directly related to the apparent decrease of u, d quark masses when unquenching QCD lattice simulations. The y parameter can be estimated from a semi-quenched analysis, in which there are no strange quarks in the sea, the result being y = 0.59(13). [S0556-2821(99)02205-5].

We present results for the b \bar b spectrum obtained using an O(M_bv^6)-correct non-relativistic lattice QCD action, where M_b denotes the bare b-quark mass and v^2 is the mean squared quark velocity. Propagators are evaluated on SESAM's three sets of dynamical gauge configurations generated with two flavours of Wilson fermions at beta = 5.6. These results, the first of their kind obtained with dynamical Wilson fermions, are compared to a quenched analysis at equivalent lattice spacing, beta = 6.0. Using our three sea-quark values we perform the ``chiral'' extrapolation to m_eff = m_s/3, where m_s denotes the strange quark mass. The light quark mass dependence is found to be small in relation to the statistical errors. Comparing the full QCD result to our quenched simulation we find better agreement of our dynamical data with experimental results in the spin-independent sector but observe no unquenching effects in hyperfine-splittings. To pin down the systematic errors we have also compared quenched results in different ``tadpole'' schemes as well as using a lower order action. We find that spin-splittings with an O(M_bv^4) action are O(10%) higher compared to O(M_bv^6) results. Relative to the results obtained with the plaquette method the Landau gauge mean link tadpole scheme raises the spin splittings by about the same margin so that our two improvements are opposite in effect.

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(pi)a > (0.23L)(-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 T chi L 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 threefold: (i) we demonstrate that algorithmic improvements like fast Krylov solvers and parallel preconditioning recently introduced can be put into practise in full QCD simulations, (ii) we present encouraging observations as to the critical dynamics of the Hybrid Monte Carlo algorithm in the approach to the chiral limit, (iii) we mention signal improvements of noisy estimator techniques for disconnected diagrams to the pi-N sigma term, and (iv) we report on SESAM's results for light hadron spectrum, light quark masses, and heavy quarkonia.

We estimate the strong coupling constant from the perturbative expansion of the plaquette. The scale is set by the 2S-1S and 1P-1S splittings in bottomonium which are computed in NRQCD on dynamical gauge configurations with nf=2 degenerate Wilson quarks at intermediate masses. We have increased the statistics of our spectrum calculation in order to reliably extrapolate in the sea-quark mass. We find a value of alpha_MS(m_Z) = 0.1118(26) which is somewhat lower than previous estimates within NRQCD.

We present results on the static potential, and torelon and glueball masses from simulations of QCD with two flavours of dynamical Wilson fermions on 163 × 32 and 243 × 40 lattices at β = 5.6.

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 large QCD vacuum field ensembles (with 3000 to 5000 trajectories each and m(pi)/m(rho)- ratios in the range greater than or equal to 0.69, for two sea quark flavors), we are able to establish 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 such as the spin content of the proton, or the eta' mass. [S0556-2821(98)50319-0]

Disconnected diagrams are expected to be sensitive to the inclusion of dynamical fermions. We present a feasibility study for the observation of such effects on the nucleonic matrix elements of the axial vector current, using SESAM full QCD vacuum configurations with Wilson fermions on 163 × 32 lattices, at β = 5.6. Starting from the standard methods developed by the Kentucky and Tsukuba groups, we investigate the improvement from various refinements thereof.

We investigate the critical dynamics of the Hybrid Monte Carlo algorithm approaching the chiral limit of standard Wilson fermions. Our observations are based on time series of lengths O(500) for a variety of observables. The lattice sizes are 163 × 32 and 243 × 40. We work at β = 5.6, and κ = 0.156, 0.157, 0.1575, 0.158, with 0.83 > mπmϱ > 0.55. We find surprisingly small integrated autocorrelation times for local and extended observables. The dynamical critical exponent z of the exponential autocorrelation time is compatible with 2. We estimate the total computational effort to scale between V2and V214 toward the chiral limit.

We present results for the bb spectrum obtained using an O(Mbv6)-correct non-relativistic lattice QCD action. Propagators are evaluated on SESAM's three sets of dynamical gauge configurations generated with two flavours of Wilson fermions at β = 5.6. Compared to a quenched simulation at equivalent lattice spacing we find better agreement of our dynamical data with experimental results in the spin-independent sector but observe no unquenching effects in hyperfine-splittings. To pin down the systematic errors we have also compared quenched results in different “tadpole” schemes and used a lower order action.

We calculate the masses of light and strange quarks using a high statistics lattice simulation of QCD with nf = 2 dynamical Wilson fermions. The simulation has been carried out at three different values of the sea quark mass. For each sea quark we have analyzed our data at five different values of the valence quark mass. This enables us to parameterize our results successfully in the (mseaq,mvalenceq) plane, using a linear ansatz according to first order chiral perturbation theory (CPBT). We find mligtMS(2GeV) = 2.7(2)MeV, which is lower than the result found in quenched simulations. For the strange quark, in a sea of two dynamical light quarks, we obtain mstrangeMS(2GeV) = 140(20)MeV.

In a full QCD lattice study with Nf = 2 Wilson fermions, we seek to optimize the signals for the disconnected contributions to the Pion Nucleon Sigma term σN. We demonstrate, in form of a numerical analysis, that — in order to achieve a tolerable signal to noise-ratio in full QCD - it is advantageous to work with a Z2-noise source rather than to use Gaussian noise sources or to rely only on gange invariance to cancel non-gauge-invariant background. We find that 10 Z2-noise sources suffice on our sample (about 150 independent QCD configurations at β = 5.6 on 163 × 32 with κsea = 0.157, equivalent to MπMϱ = 0.7(1), to achieve decent signals and adequate fluctuations, rather than 300 such sources as recently used in quenched simulations.

We present recent results of SESAM's large scale lattice simulation of QCD with two dynamical flavours of Wilson fermions. The emphasis is on future prospects in the extraction of flavour-singlet matrix elements, i.e the pi-nucleon sigma term, with dynamical configurations.

Disconnected diagrams are expected to be sensitive to the inclusion of dynamical fermions. We present a feasibility study for the observation of such effects on the nucleonic matrix elements of the axial vector current, using SESAM full QCD vacuum configurations with Wilson fermions on 16 × 32 lattices, at β = 5.6. Starting from the standard methods developed by the Kentucky and Tsukuba groups, we investigate the improvement from various refinements thereof.

We determine the masses of the light and the strange quarks in the MS-scheme using our high-statistics lattice simulation of QCD with dynamical Wilson fermions. For the light quark mass we find mlightMS (2GeV) = 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 mstrangeMS(2GeV) = 140(20)MeV.

We present results for the b ¯ b spectrum obtained using an O(M b v 6)-correct non-relativistic lattice QCD action. Propagators are evaluated on SESAM's three sets of dynamical gauge configurations generated with two flavours of Wilson fermions at β = 5.6. Compared to a quenched simulation at equivalent lattice spacing we find better agreement of our dynamical data with experimental results in the spin-independent sector but observe no unquenching effects in hyperfine-splittings. To pin down the systematic errors we have also compared quenched results in different " tadpole " schemes and used a lower order action.

We present results for spectroscopy, quark masses and decay constants obtained from SESAM's and TχL's large statistics simulations of QCD with two dynamical Wilson fermions.