Search

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides., Comment: 5 pages, subimtted to Physical Review Letters

The 197Au(gamma,n) reaction is used as an activation standard for photodisintegration studies on astrophysically relevant nuclei. At the bremsstrahlung facility of the superconducting electron accelerator ELBE (Electron Linear accelerator of high Brilliance and low Emittance) of Forschungszentrum Dresden-Rossendorf, photoactivation measurements on 197Au have been performed with bremsstrahlung endpoint energies from 8.0 to 15.5 MeV. The measured activation yield is compared with previous experiments as well as with calculations using Hauser-Feshbach statistical models. It is shown that the experimental data are best described by a two-Lorentzian parametrization with taking the axial deformation of 197Au into account. The experimental 197Au(gamma,n) reaction yield measured at ELBE via the photoactivation method is found to be consistent with previous experimental data using photon scattering or neutron detection methods., Comment: 9 pages

The dipole response of the N=50 nucleus 90Zr was studied in photon-scattering experiments at the electron linear accelerator ELBE with bremsstrahlung produced at kinetic electron energies of 7.9, 9.0, and 13.2 MeV. We identified 189 levels up to an excitation energy of 12.9 MeV. Statistical methods were applied to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. In this way we derived the photoabsorption cross section up to the neutron-separation energy. This cross section matches well the photoabsorption cross section obtained from (gamma,n) data and thus provides information about the extension of the dipole-strength distribution toward energies below the neutron-separation energy. An enhancement of E1 strength has been found in the range of 6 MeV to 11 MeV. Calculations within the framework of the quasiparticle-phonon model ascribe this strength to a vibration of the excessive neutrons against the N = Z neutron-proton core, giving rise to a pygmy dipole resonance., Comment: 27 pages, 15 figures, to be published in PRC

In explosive stellar environments like supernovae, the temperatures are high enough for the production of heavy neutron-deficient nuclei, the socalled p-nuclei. Up to now, the knowledge of the reaction rates of p-nuclei is based on theoretical parameterizations using statistical model calculations. At the bremsstrahlung facility of the superconducting electron accelerator ELBE of FZ Dresden-Rossendorf, we aim to measure the photodisintegration rates of heavy nuclei experimentally. Photoactivation measurements on the astrophysically relevant p-nuclei 92Mo and 144Sm have been performed with bremsstrahlung end-point energies from 10.0 to 16.5 MeV. First experiments on the short-lived decays following the reaction 144Sm(gamma,n) are carried out using a pneumatic delivery system for rapid transport of activated samples. The activation yields are compared with calculations using cross sections from recent Hauser-Feshbach models., Comment: 6 pages, 3 figures

Electromagnetic dipole-strength distributions up to the particle separation energies are studied for the stable even-even nuclides $^{92,94,96,98,100}$Mo in photon scattering experiments at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf. The influence of inelastic transitions to low-lying excited states has been corrected by a simulation of $\gamma$ cascades using a statistical model. After corrections for branching ratios of ground-state transitions, the photon-scattering cross-sections smoothly connect to data obtained from $(\gamma,n)$-reactions. With the newly determined electromagnetic dipole response of nuclei well below the particle separation energies the parametrisation of the isovector giant-dipole resonance is done with improved precision., Comment: Proceedings Nuclear Physics in Astrophysics 3, March 2007, Dresden Journal of Physics G, IOP Publishing

A research program has been started to study experimentally the near-threshold photodissociation of nuclides in the chain of cosmic heavy element production with bremsstrahlung from the ELBE accelerator. An important prerequisite for such studies is good knowledge of the bremsstrahlung distribution which was determined by measuring the photodissociation of the deuteron and by comparison with model calculations. First data were obtained for the astrophysically important target nucleus 92-Mo by observing the radioactive decay of the nuclides produced by bremsstrahlung irradiation at end-point energies between 11.8 MeV and 14.0 MeV. The results are compared to recent statistical model calculations., Comment: 6 pages, 8 figures, Proceedings Nuclear Physics in Astrophysics II, May 16-20, 2005, Debrecen, Hungary. The original publication is available at www.eurphysj.org

As computing resources are limited, choosing the parameters for a full Lattice QCD simulation always amounts to a compromise between the competing objectives of a lattice spacing as small, quarks as light, and a volume as large as possible. Aiming to push unquenched simulations with the Wilson action towards the computationally expensive regime of small quark masses we address the question whether one can possibly save computing time by extrapolating results from small lattices to the infinite volume, prior to the usual chiral and continuum extrapolations. In the present work the systematic volume dependence of simulated pion and nucleon masses is investigated and compared with a long-standing analytic formula by Luescher and with results from Chiral Perturbation Theory. We analyze data from Hybrid Monte Carlo simulations with the standard (unimproved) two-flavor Wilson action at two different lattice spacings of a=0.08fm and 0.13fm. The quark masses considered correspond to approximately 85 and 50% (at the smaller a) and 36% (at the larger a) of the strange quark mass. At each quark mass we study at least three different lattices with L/a=10 to 24 sites in the spatial directions (L=0.85-2.08fm)., Comment: 21 pages, 20 figures, REVTeX 4; v2: caption of Fig.7 corrected, one reference added

The extreme computational costs of calculating the sign of the Wilson matrix within the overlap operator have so far prevented four dimensional dynamical overlap simulations on realistic lattice sizes, because the computational power required to invert the overlap operator, the time consuming part of the Hybrid Monte Carlo algorithm, is too high. In this series of papers we introduced the optimal approximation of the sign function and have been developing preconditioning and relaxation techniques which reduce the time needed for the inversion of the overlap operator by over a factor of four, bringing the simulation of dynamical overlap fermions on medium-size lattices within the range of Teraflop-computers. In this paper we adapt the HMC algorithm to overlap fermions. We approximate the matrix sign function using the Zolotarev rational approximation, treating the smallest eigenvalues of the Wilson operator exactly within the fermionic force. We then derive the fermionic force for the overlap operator, elaborating on the problem of Dirac delta-function terms from zero crossings of eigenvalues of the Wilson operator. The crossing scheme proposed shows energy violations which are better than O($\Delta\tau^2$) and thus are comparable with the violations of the standard leapfrog algorithm over the course of a trajectory. We explicitly prove that our algorithm satisfies reversibility and area conservation. Finally, we test our algorithm on small $4^4$, $6^4$, and $8^4$ lattices at large masses., Comment: v2 60 pages; substantial changes to all parts of the article; v3 minor revsions

A complete description of the nucleon structure in terms of generalized parton distributions (GPDs) at twist 2 level requires the measurement/computation of the eight functions H, E, \tilde H, \tilde E, H_T, E_T, \tilde H_T and \tilde E_T, all depending on the three variables x, \xi and t. In this talk, we present and discuss our first steps in the framework of lattice QCD towards this enormous task. Dynamical lattice QCD results for the lowest three Mellin moments of the helicity dependent and independent GPDs are shown in terms of their corresponding generalized form factors. Implications for the transverse coordinate space structure of the nucleon as well as the orbital angular momentum (OAM) contribution of quarks to the nucleon spin are discussed in some detail., Comment: 5 pages, 5 figures, Talk presented by Ph.H. at Electron-Nucleus Scattering VIII, Elba, Italy, June 21-25, 2004; typos corrected, minor change in wording on p.4&5

We present results of our ongoing determination of string breaking in full QCD with N_f=2 Wilson fermions. Our investigation of the fission of the static quark-antiquark string into a static-light meson-antimeson system is based on dynamical configurations of size 24^3 x 40 produced by the TxL collaboration. Combining various optimization methods we determine the matrix elements of the two-by-two system with so far unprecedented accuracy. The all-to-all light quark propagators occurring in the transition element are computed from eigenmodes of the Hermitian Wilson-Dirac matrix complemented by stochastic estimates in the orthogonal subspace. We observe a clear signature for level-splitting between ground state and excited potential. Thus, for the first time, string breaking induced by sea quarks is observed in a simulation of 4-dimensional lattice-QCD., Comment: 3 pages, 3 figures, contribution to Lattice 2004

We apply the Hybrid Monte Carlo method to the simulation of overlap fermions. We give the fermionic force for the molecular dynamics update. We present early results on a small dynamical chiral ensemble., Comment: Lattice2004(machines), 3 pages; references updated, minor changes to text

This talk presents recent calculations in full QCD of the lowest three moments of generalized parton distributions and the insight they provide into the behavior of nucleon electromagnetic form factors, the origin of the nucleon spin, and the transverse structure of the nucleon. In addition, new exploratory calculations in the chiral regime of full QCD are discussed., Comment: 9 pages, 9 figures, Cairns Workshop on Lattice Hadron Physics

It has been known for a long time that the large experimental singlet-octet mass gap in the pseudoscalar meson mass spectrum originates from the anomaly of the axial vector current, i.e. from nonperturbative effects and the nontrivial topological structure of the QCD vacuum. In the N_colour -> infinity limit of the theory, this connection elucidates in the famous Witten-Veneziano relation between the eta'-mass and the topological susceptibility of the quenched QCD vacuum.While lattice QCD has by now produced impressive high precision results on the flavour nonsinglet hadron spectrum, the determination of the pseudoscalar singlet mesons from direct correlator studies is markedly lagging behind, due to the computational complexity in handling observables that include OZI-rule violating diagrams, like the eta' propagator. In this article, we report on some recent progress in dealing with the numerical bottleneck problem., Comment: 29 pages, 20 figures, Introductory review based on a talk presented by K. Schilling at the 1st Workshop on Lattice Hadron Physics, Cairns, Australia

We report on a first, comprehensive partially quenched study of the eta-eta' problem, based on SESAM configurations on a 16^3x32 lattice at beta=5.6 QCD with two (mass degenerate) active sea quark flavours. By means of the spectral approximation of the two-loop (hairpin) diagrams, we find clear plateau formation in the effective masses which enables us both to determine the eta-eta' mass matrix and the alpha-parameter in the effective chiral Lagrangian for the flavour singlet sector, alpha=0.028 +- 0.013., Comment: 3 pages, 5 figures, contribution to Lattice2003

This work presents the first calculation in lattice QCD of three moments of spin-averaged and spin-polarized generalized parton distributions in the proton. It is shown that the slope of the associated generalized form factors decreases significantly as the moment increases, indicating that the transverse size of the light-cone quark distribution decreases as the momentum fraction of the struck parton increases., Comment: 4 pages, 1 figure

The aim of the GRAL project is to simulate full QCD with standard Wilson fermions at light quark masses on small to medium-sized lattices and to obtain infinite-volume results by extrapolation. In order to establish the functional form of the volume dependence we study systematically the finite-size effects in the light hadron spectrum. We give an update on the status of the GRAL project and show that our simulation data for the light hadron masses depend exponentially on the lattice size., Comment: 3 pages, 1 figure, Lattice2003(spectrum)

The lowest three moments of generalized parton distributions are calculated in full QCD and provide new insight into the behavior of nucleon electromagnetic form factors, the origin of the nucleon spin, and the transverse structure of the nucleon., Comment: 3 pages, Lattice2003(Theoretical developments)

We present a lattice measurement of the first two moments of the spin-dependent GPD H-tilde(x,xi,t). From these we obtain the axial coupling constant and the second moment of the spin-dependent forward parton distribution. The measurements are done in full QCD using Wilson fermions. In addition, we also present results from a first exploratory study of full QCD using Asqtad sea and domain-wall valence fermions., Comment: Lattice2003(Theory), 3 pages, 3 figures, to appear in the Proceedings of Lattice 2003

Calculations of the magnetic dipole, electric quadrupole and Coulomb quadrupole amplitudes for the transition $\gamma N\to \Delta$ are presented both in quenched QCD and with two flavours of degenerate dynamical quarks., Comment: Lattice2003(Matrix), 3 pages

The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition $\gamma N\to \Delta$ are evaluated both in quenched lattice QCD at $\beta=6.0$ and using two dynamical Wilson fermions simulated at $\beta=5.6$. The dipole transition form factor is accurately determined at several values of momentum transfer. On the lattices studied in this work, the electric quadrupole amplitude is found to be non-zero yielding a negative value for the ratio, $ R_{EM}$, of electric quadrupole to magnetic dipole amplitudes at three values of momentum transfer., Comment: Discussion of systematic errors and chiral extrapolation extended; Typos corrected; Version to appear in Phys. Rev. D.; 19 pages including 21 figures

Many appplications in computational science are sufficiently compute-intensive that they depend on the power of parallel computing for viability. For all but the "embarrassingly parallel" problems, the performance depends upon the level of granularity that can be achieved on the computer platform. Our computational particle physics applications require machines that can support a wide range of granularities, but in general, compute-intensive state-of-the-art projects will require finely grained distributions. Of the different types of machines available for the task, we consider cluster computers. The use of clusters of commodity computers in high performance computing has many advantages including the raw price/performance ratio and the flexibility of machine configuration and upgrade. Here we focus on what is usually considered the weak point of cluster technology; the scaling behaviour when faced with a numerically intensive parallel computation. To this end we examine the scaling of our own applications from numerical quantum field theory on a cluster and infer conclusions about the more general case., Comment: 26pp. LaTeX2e using package graphicx. 16 PostScript figures

Moments of light cone quark density, helicity, and transversity distributions are calculated in unquenched lattice QCD at $\beta = 5.5$ and $\beta = 5.3$ using Wilson fermions on $ 16^3 \times 32 $ lattices. These results are combined with earlier calculations at $\beta = 5.6$ using SESAM configurations to study the continuum limit., Comment: Lattice 2002 (hadronic matrix elements) 3 pages, 1 figure

The fomalism is developed to express nucleon matrix elements of the electromagnetic current in terms of form factors consistent with the translational, rotational, and parity symmetries of a cubic lattice. We calculate the number of these form factors and show how appropriate linear combinations approach the continuum limit., Comment: Lattice 2002 (hadronic matrix elements) 3 pages

We report new results from our finite size scaling analysis of 4d compact pure U(1) gauge theory with Wilson action. Investigating several cumulants of the plaquette energy within the Borgs-Kotecky finite size scaling scheme we find strong evidence for a first-order phase transition and present a high precision value for the critical coupling in the thermodynamic limit., Comment: Lattice2002(Spin)

We present new results from our ongoing study of flavor singlet pseudoscalar mesons in QCD. Our approach is based on (a) performing truncated eigenmode expansions for the hairpin diagram and (b) incorporating the ground state contribution for the connected meson propagator. First, we explain how the computations can be substantially improved by even-odd preconditioning. We extend previous results on early mass plateauing in the eta' channel of two-flavor full QCD with degenerate sea and valence quarks to the partially quenched situation. We find that early mass plateau formation persists in the partially quenched situation., Comment: Lattice2002(spectrum), 3 pages, 5 figures

We present results on the ratio of electric quadrupole to magnetic dipole amplitudes, $R_{EM}={\cal G}_{E2}/{\cal G}_{M1}$, for the transition $\gamma N to \Delta$ from lattice QCD. We consider both the quenched and the 2-flavor theory., Comment: 3 pages, 4 figures, talk presented at Lattice2002(matrixel); Layout of figures adjusted

The numerical and computational aspects of the overlap formalism in lattice quantum chromodynamics are extremely demanding due to a matrix-vector product that involves the sign function of the hermitian Wilson matrix. In this paper we investigate several methods to compute the product of the matrix sign-function with a vector, in particular Lanczos based methods and partial fraction expansion methods. Our goal is two-fold: we give realistic comparisons between known methods together with novel approaches and we present error bounds which allow to guarantee a given accuracy when terminating the Lanczos method and the multishift-CG solver, applied within the partial fraction expansion methods., Comment: 30 pages, 2 figures

Moments of the quark density, helicity, and transversity distributions are calculated in unquenched lattice QCD. Calculations of proton matrix elements of operators corresponding to these moments through the operator product expansion have been performed on $16^3 \times 32$ lattices for Wilson fermions at $\beta = 5.6$ using configurations from the SESAM collaboration and at $\beta = 5.5$ using configurations from SCRI. One-loop perturbative renormalization corrections are included. At quark masses accessible in present calculations, there is no statistically significant difference between quenched and full QCD results, indicating that the contributions of quark-antiquark excitations from the Dirac Sea are small. Close agreement between calculations with cooled configurations containing essentially only instantons and the full gluon configurations indicates that quark zero modes associated with instantons play a dominant role. Naive linear extrapolation of the full QCD calculation to the physical pion mass yields results inconsistent with experiment. Extrapolation to the chiral limit including the physics of the pion cloud can resolve this discrepancy and the requirements for a definitive chiral extrapolation are described., Comment: 53 Pages Revtex, 26 Figures, 9 Tables. Added additional reference and updated referenced data in Table IX

We apply spectral methods to compute the OZI-rule suppressed loop-loop correlators in the pseudoscalar meson flavour singlet channel. Using SESAM configurations (obtained with two degenerate sea quark flavours on 16x16x16x32 lattices at beta = 5.6 with standard Wilson action), we find for the first time clear evidence for mass plateau formation in the eta' channel of this theory. As a consequence, we observe a clear signal of a mass gap persistent under chiral extrapolation. This sets the stage for a more realistic two-channel approach, where partially quenched strange quarks would be included, in addition to u, d sea quarks., Comment: 5 pages, 8 Postscript figures

In order to construct improved overlap fermions, we start from a short ranged approximate Ginsparg-Wilson fermion and insert it into the overlap formula. We show that its polynomial evaluation is accelerated considerably compared to the standard Neuberger fermion. In addition the degree of locality is strongly improved., Comment: Talk presented at Lattice 2001 (improvement), 3 pages, 3 figures

Improvements of various methods to compute the sign function of the hermitian Wilson-Dirac matrix within the overlap operator are presented. An optimal partial fraction expansion (PFE) based on a theorem of Zolotarev is given. Benchmarks show that this PFE together with removal of converged systems within a multi-shift CG appears to approximate the sign function times a vector most efficiently. A posteriori error bounds are given., Comment: 3 pages, poster contribution to Lattice2001(algorithms)

We present a report on the status of the GRAL project (Going Realistic And Light), which aims at simulating full QCD with two dynamical Wilson quarks below the vector meson decay threshold, m_ps/m_v < 0.5, making use of finite-size-scaling techniques., Comment: 3 pages, 6 figures, Lattice2001(spectrum)

The non-normality of Wilson-type lattice Dirac operators has important consequences - the application of the usual concepts from the textbook (hermitian) quantum mechanics should be reconsidered. This includes an appropriate definition of observables and the refinement of computational tools. We show that the truncated singular value expansion is the optimal approximation to the inverse operator D^{-1} and we prove that due to the gamma_5-hermiticity it is equivalent to gamma_5 times the truncated eigenmode expansion of the hermitian Wilson-Dirac operator., Comment: Lattice2001(theorydevelop)

We present new data on the mass of the light and strange quarks from SESAM/T$\chi$L. The results were obtained on lattice-volumes of $16^3\times 32$ and $24^3\times 40$ points, with the possibility to investigate finite-size effects. Since the SESAM/T$\chi$L ensembles at $\beta=5.6$ have been complemented by configurations with $\beta=5.5$, moreover, we are now able to attempt the continuum extrapolation (CE) of the quark masses with standard Wilson fermions., Comment: Lattice2001(spectrum), minor corrections

We perform an eta' mass analysis based on a total of 1130 dynamical gauge field configurations, with 5 different quark mass values on lattices of size 16^3*32 (SESAM) and 24^3*40 (TkL) at beta = 5.6. We employ the stochastic estimator technique and spectral methods to deal with the disconnected piece of the flavour singlet correlation function. We demonstrate that very early plateau formation in the local eta' mass can be achieved by first ground state projecting the connected piece of its correlator., Comment: Lattice(spectrum)

As part of a systematic algorithm study, we present first results on a performance comparison between a multibosonic algorithm and the hybrid Monte Carlo algorithm as employed by the SESAM collaboration. The standard Wilson fermion action is used on 32*16^3 lattices at beta=5.5., Comment: LaTeX, 3 pages, Lattice2001(algorithms)

We demonstrate the utility of a spectral approximation to fermion loop operators using low-lying eigenmodes of the hermitian Dirac-Wilson matrix, Q. The investigation is based on a total of 400 full QCD vacuum configurations, with two degenerate flavors of dynamical Wilson fermions at beta =5.6, at two different sea quark masses. The spectral approach is highly competitive for accessing both topological charge and disconnected diagrams, on large lattices and small quark masses. We propose suitable partial summation techniques that provide sufficient saturation for estimating Tr Q^{-1}, which is related to the topological charge. In the effective mass plot of the eta' meson we achieved a consistent early plateau formation, by ground state projecting the connected piece of its propagator., Comment: 15 pages, 25 figures, citations added

We consider the local chirality of near-zero eigenvectors from Wilson-Dirac and clover improved Wilson-Dirac lattice operators as proposed recently by Horv\'ath et al. We studied finer lattices and repaired for the loss of orthogonality due to the non-normality of the Wilson-Dirac matrix. As a result we do see a clear double peak structure on lattices with resolutions higher than 0.1 fm. We found that the lattice artifacts can be considerably reduced by exploiting the biorthogonal system of left and right eigenvectors. We conclude that the dominance of instantons on topological charge fluctuations is not ruled out by local chirality measurements., Comment: 10 pages, 6 figures

We describe results of a high-statistics finite size scaling analysis of 4d compact U(1) lattice gauge theory with Wilson action at the phase transition point. Using a multicanonical hybrid Monte Carlo algorithm we generate data samples with more than 150 tunneling events between the metastable states of the system, on lattice sizes up to 18^4. We performed a first analysis within the Borgs-Kotecky finite size scaling scheme. As a result, we report evidence for a first-order phase transition with a plaquette energy gap, G=0.02667(20), at a transition coupling, beta_T=1.011128(11)., Comment: Lattice 2000 (Topics in Gauge Theories),6 pages, 6 figures, LaTeX

Moments of the quark density distribution, moments of the quark helicity distribution, and the tensor charge are calculated in full QCD. Calculations of matrix elements of operators from the operator product expansion have been performed on $16^3 \times 32$ lattices for Wilson fermions at $\beta = 5.6$ using configurations from the SESAM collaboration and at $\beta = 5.5$ using configurations from SCRI. One-loop perturbative renormalization corrections are included. Selected results are compared with corresponding quenched calculations and with calculations using cooled configurations., Comment: Lattice 2000 (Hadronic Matrix Elements), 4 pages, 5 figures

We describe an explicit construction of approximate Ginsparg-Wilson fermions for QCD. We use ingredients of perfect action origin, and further elements. The spectrum of the lattice Dirac operator reveals the quality of the approximation. We focus on beta =6 for optimisation. Such fermions are intended to be inserted into the overlap formula. Hence we also test the speed of convergence under polynomial evaluation of the overlap formula., Comment: 5 pages, poster presented at Lattice 2000 (Improvement and Renormalisation)

We perform a lattice mass analysis in the flavour singlet pseudoscalar channel on the SESAM and TXL full QCD vacuum configurations, with 2 active flavours of dynamical Wilson fermions at beta = 5.6. At our inverse lattice spacing, a^-1 = 2.3 GeV, we retrieve by a chiral extrapolation to the physical light quark masses the value m_eta' = 3.7(+8)(-4) m_pi. A crude extrapolation from (N_f = 3) phenomenology would suggest m_eta' \approx 5.1 m_pi for N_f = 2 QCD. we verify that the mass gap between the singlet state eta' and the pi flavour triplt state is due to gauge configurations with non-trivial topology., Comment: 8 pages, 10 figures

We present a study on the direct determination of the $\eta '$ mass on the full set of SESAM and T$\chi$L QCD vacuum configurations with 2 active flavours of Wilson fermions, at $\beta = 5.6$. We observe a definite dependency of the two-loop correlator on the topological charge sector., Comment: 10 pages

For lattice QCD with two sea quark flavours we compute the static quark antiquark potential V(R) in the regime where string breaking is expected. In order to increase statistics, we make full use of the lattice information by including all lattice vectors R to any possible lattice separation in the infrared regime. The corresponding paths between the lattice points are constructed by means of a generalized Bresenham algorithm as known from computer graphics. As a results we achieve a determination of the unquenched potential in the range .8 to 1.5 fm with hitherto unknown precision. Furthermore, we demonstrate some error reducing methods for the evaluation of the transition matrix element between two- and four-quark states., Comment: 6 pages, 7 figures

Considerable progress has been achieved recently in the observation of string breaking within non-Abelian Higgs models, by use of multi-channel methods allowing for broken string states. Similarly, in pure gauge theory this approach has been shown to reveal string breaking for color charges in the adjoint represaentation. For QCD with dynmical fermions, one needs substantial progress in noise reduction, however, to render such techniques viable., Comment: June 1999, 6 pages, 11 figures. Plenary talk presented at the 17th Int. Symposium on Lattice Field Theory (Lattice 99), Pisa, Italy 29 Jun - 3 Jul, 1999

It is shown that the 21264 Alpha processor can reach about 20% sustained efficiency for the inversion of the Wilson-Dirac operator. Since fast ethernet is not sufficient to get balancing between computation and communication on reasonable lattice- and system-sizes, an interconnection using Myrinet is discussed. We find a price/performance ratio comparable with state-of-the-art SIMD-systems for lattice QCD., Comment: LATTICE99(machines), 3 pages