P. Ueberholz | Niederrhein University of Applied Sciences (original) (raw)

Papers by P. Ueberholz

Physical Review D, 1998

We address a long standing issue and determine the decorrelation efficiency of the Hybrid Monte C... more 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 mπ mρ-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 η ′ mass.

We calculate connected and disconnected contributions to the flavour singlet scalar density ampli... more We calculate connected and disconnected contributions to the flavour singlet scalar density amplitude of the nucleon in a full QCD lattice simulation with n_f=2 dynamical Wilson fermions at β=5.6 on a 16^3 × 32 lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate σ_π 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).

We present the results of the first lattice study of semileptonic decays of baryons containing a ... more We present the results of the first lattice study of semileptonic decays of baryons containing a b-quark. Predictions for the decay distributions are given and the Isgur-Wise functions for heavy baryons are computed, for values of the velocity transfer up to about ω = 1.2. The computations are performed on a 24 3 ×48 lattice at β = 6.2 using the Sheikholeslami-Wohlert action in the quenched approximation.

Abstract. Computing physical quantities in lattice quantum chromodynamics means solving huge syst... more Abstract. Computing physical quantities in lattice quantum chromodynamics means solving huge systems of linear equations (O(10 7) equations). Efficient parallel Krylov subspace solvers play a vital role in the solution of these systems. We present a detailed analysis of the performance of the stabilized biconjugate gradient algorithm with preconditioning on massively parallel CRAY T3E systems. 1 Lattice Gauge Theory computations The numerical investigation of quantum chromodynamics (QCD) on a four-dimensional space-time grid is one of the grand challenges in high-performance scientific computing [1, 2]. QCD is considered to be the fundamental theory of strongly interacting particles. After 20 years of research, the strong coupling regime of QCD [3] still has not been solved in a non-perturbative analytical approach, and it is by now widely believed that the numerical treatment of the theory on the lattice using very fast parallel supercomputers is the only viable scheme to extract q...

We estimate the QCD coupling constant from a lattice calculation of the bottomonium spectrum. The... more We estimate the QCD coupling constant from a lattice calculation of the bottomonium spectrum. The second order perturbative expansion of the pla-quette expectation value is employed to determine α 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 flavours 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 flavours to find a value of α (5) MS (m Z) = 0.1118(17). The error quoted covers both statistical and systematic uncertainties in the scale determination. An additional 5% uncertainty 1 comes from the choice of the underlying sea quark formulation and from trun-cation errors in perturbative expansions.

arXiv: Condensed Matter, 1993

The evolution equation for the expectation values of the Boltzmann factor between valence bond st... more The evolution equation for the expectation values of the Boltzmann factor between valence bond states is evaluated in lowest order of the dimer cluster expansion. Explicit formulas are given for the internal energy and the specific heat of the d-dimensional antiferromagnetic Heisenberg model.

Disconnected diagrams are expected to be sensitive to the inclusion of dynamical fermions. We pre... more 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.

Heat Transfer Engineering

International Journal of Engineering Systems Modelling and Simulation

Processing of large data sets with high through put is one of the major focus of Grid computing t... more Processing of large data sets with high through put is one of the major focus of Grid computing today. If possible, data are split up into small chunks that are processed independently. Thus, job sets of hundreds > or even thousands of individual jobs are possible. For the job submitter or the resource providers such a scenario is a nightmare currently, as it is hard to keep track of such an amount of jobs or to identify failure reasons. We present a system that will support gLite users to track and monitor their jobs and their resource usage, to nd and identify failure reasons and even to steer running applications.

Physical Review D, 2001

For lattice QCD with two sea quark flavours we compute the static quark antiquark potential V (R)... more 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 given separation R = |R| in the infrared regime. The corresponding paths between the lattice points are constructed by means of a generalized Bresenhamalgorithm as known from computer graphics. As a result, we achieve a determination of the unquenched potential in the range 0.8 to 1.5 fm with hitherto unknown precision. Furthermore, we demonstrate some error reducing methods for the evaluation of the transition matrix element between twoand four-quark states.

Nuclear Physics B - Proceedings Supplements, 1995

We study a topological updating scheme in three dimensional U(1) gauge theory. Some expectations ... more We study a topological updating scheme in three dimensional U(1) gauge theory. Some expectations for four dimensional SU(N) gauge theories are discussed.

Nuclear Physics B - Proceedings Supplements, 1993

In numerical simulations, compact quantum electrodynamics exhibits a weakly first-order phase tra... more In numerical simulations, compact quantum electrodynamics exhibits a weakly first-order phase transition. On large lattice sizes, using conventional updating algorithms, this goes along with strong metastabilities near the location of the phase transition. The relative weight of the two phases cannot be marked out accurately enough within a reasonable amount of simulation time. We discuss a phenomenological method that allows to determine the relative weight between the phases in principle with great precision. Our method is expected to give reliable results for any system exhibiting a similar phase transition pattern.

Applied and Environmental Microbiology, 2000

Neutral red (NR) was utilized as an electron mediator in microbial fuel cells consuming glucose t... more Neutral red (NR) was utilized as an electron mediator in microbial fuel cells consuming glucose to study both its efficiency during electricity generation and its role in altering anaerobic growth and metabolism of Escherichia coli and Actinobacillus succinogenes. A study of chemical fuel cells in which NADH, NR, and ferricyanide were the electron donor, the electronophore, and the electron acceptor, respectively, showed that electrical current produced from NADH was proportional to the concentration of NADH. Fourfold more current was produced from NADH in chemical fuel cells when NR was the electron mediator than when thionin was the electron mediator. In microbial fuel cells in which E. coli resting cells were used the amount of current produced from glucose when NR was the electron mediator (3.5 mA) was 10-fold more than the amount produced when thionin was the electron mediator (0.4 mA). The amount of electrical energy generated (expressed in joules per mole of substrate) and the amount of current produced from glucose (expressed in milliamperes) in NR-mediated microbial fuel cells containing either E. coli or A. succinogenes were about 10-and 2-fold greater, respectively, when resting cells were used than when growing cells were used. Cell growth was inhibited substantially when these microbial fuel cells were making current, and more oxidized end products were formed under these conditions. When sewage sludge (i.e., a mixed culture of anaerobic bacteria) was used in the fuel cell, stable (for 120 h) and equivalent levels of current were obtained with glucose, as observed in the pureculture experiments. These results suggest that NR is better than other electron mediators used in microbial fuel cells and that sludge production can be decreased while electricity is produced in fuel cells. Our results are discussed in relation to factors that may improve the relatively low electrical efficiencies (1.2 kJ/mol) obtained with microbial fuel cells.

Proc. Cracow …, 2006

To process large amounts of data produced at the Large Hadron Col-lider at CERN, users typically ... more To process large amounts of data produced at the Large Hadron Col-lider at CERN, users typically submit hundreds or thousands of jobs. This requires that users can keep track of their single jobs. In this pa-per, we present a user-centric system to monitor jobs, ...