arXiv subjects
Publications and source records attributed to K. Schilling.
We present a study on the direct determination of the $η'$ mass on the full set of SESAM and T$χ$L QCD vacuum configurations with 2 active flavours of Wilson fermions, at $β= 5.6$. We observe a definite dependency of the two-loop correlator on the topological charge sector.
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.
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.
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.
We demonstrate that substantial progress can be achieved in the study of the phase structure of 4-dimensional compact QED by a joint use of hybrid Monte Carlo and multicanonical algorithms, through an efficient parallel implementation. This is borne out by the observation of considerable speedup of tunnelling between the metastable states, close to the phase transition, on the Wilson line. Our approach leads to a general parallelization scheme for the efficient stochastic sampling of systems where (a part of) the Hamiltonian involves the total action or energy in each update step.
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 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 alpha_{\bar MS}^{(5)}(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.
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 $β=5.6$ 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 $G_A^1 = 0.20(12)$.
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 $16^3 \times 32$ lattice. We find that both contributions are of similar size at the light quark mass. We arrive at the estimate $σ_{πN} = 18(5)$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 final analysis of the light and strange hadron spectra from a full QCD lattice simulation with two degenerate dynamical sea quark flavours at $β= 5.6$ on a $16^3 \times 32$ lattice. Four sets of sea quark masses corresponding to the range $.69 \leq m_π/m_ρ\leq .83$ are investigated. For reference we also ran a quenched simulation at $β_{\sf eff} = 6.0$, which is the point of equal lattice spacing, $a_ρ^{-1}$. 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.
We demonstrate that substantial progress can be achieved in the study of the phase structure of 4-dimensional compact QED by a joint use of hybrid Monte Carlo and multicanonical algorithms, through an efficient parallel implementation. This is borne out by the observation of considerable speedup of tunnelling between the metastable states, close to the phase transition, on the Wilson line. We estimate that the creation of adequate samples (with order 100 flip-flops) becomes a matter of half a year's runtime at 2 Gflops sustained performance for lattices of size up to 24^4.
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.
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.
In a simulation of SU(2) gauge theory we investigate, after maximal Abelian projection, the dual Maxwell equations for colour field and monopole current distributions around a static quark-antiquark pair Q_ Q in vacuo. Within the dual superconductor picture we carry out a Ginzburg-Landau type analysis of the flux tube profile. As a result we can determine the coherence length of the GL wave function related to the monopole condensate, xi = .25(3) fm, to be compared to the penetration length, lambda = >.15(2) fm (scaled with the string tension).
SSOR preconditioning of fermion matrix inversions which is parallelized using a locally-lexicographic lattice sub-division, has been shown to be very efficient for standard Wilson fermions. We demonstrate here the power of this method for the Sheikholeslami-Wohlert improved fermion action and for a renormalization group improved action incorporating couplings of the lattice fermion fields up to the diagonal in the unit hypercube.
We present results for the hybrid meson spectrum produced by gluonic excitations in full QCD using Wilson fermions. For the spin-exotic mesons with J^{PC}=1^{-+}, 0^{+-}, and 2^{+-} we find the lightest state to be 1^{-+} with a mass of 1.9(2) GeV. Results obtained for orbitally excited mesons are also presented.
We construct a locally-lexicographic SSOR preconditioner to accelerate the parallel iterative solution of linear systems of equations for two improved discretizations of lattice fermions: the Sheikholeslami-Wohlert scheme where a non-constant block-diagonal term is added to the Wilson fermion matrix and renormalization group improved actions which incorporate couplings beyond nearest neighbors of the lattice fermion fields. In case (i) we find the block llssor-scheme to be more effective by a factor about 2 than odd-even preconditioned solvers in terms of convergence rates, at beta=6.0. For type (ii) actions, we show that our preconditioner accelerates the iterative solution of a linear system of hypercube fermions by a factor of 3 to 4.
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.
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(5000) for a variety of observables. The lattice sizes are 16^3 x 32 and 24^3 x 40. We work at beta=5.6, and kappa=0.156, 0.157, 0.1575, 0.158, with 0.83 > m_pi/m_rho > 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 V^2 and V^2.25 towards the chiral limit.