328 research outputs found
Improving the dynamical overlap algorithm
We present algorithmic improvements to the overlap Hybrid Monte Carlo
algorithm, including preconditioning techniques and improvements to the
correction step, used when one of the eigenvalues of the Kernel operator
changes sign, which is now O(\Delta t^2) exact.Comment: 6 pages, 3 figures; poster contribution at Lattice 2005(Algorithms
and machines
Finite-Size Effects in Lattice QCD with Dynamical Wilson Fermions
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 adde
Monopole clusters and critical dynamics in four-dimensional U(1)
We investigate monopoles in four-dimensional compact U(1) with Wilson action.
We focus our attention on monopole clusters as they can be identified
unambiguously contrary to monopole loops. We locate the clusters and determine
their properties near the U(1) phase transition. The Coulomb phase is
characterized by several small clusters, whereas in the confined phase the
small clusters coalesce to one large cluster filling up the whole system. We
find that clusters winding around the periodic lattice are absent within both
phases and during the transition. However, within the confined phase, we
observe periodically closed monopole loops if cooling is applied.Comment: 3 pages, Wuppertal preprint WUB 93-3
Compact QED under scrutiny: it's first order
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
First-Order Signals in Compact QED with Monopole Suppressed Boundaries
Pure gauge compact QED on hypercubic lattices is considered with periodically
closed monopole currents suppressed. We compute observables on sublattices
which are nested around the centre of the lattice in order to locate regions
where translation symmetry is approximately recovered. Our Monte Carlo
simulations on -lattices give indications for a first-order nature of the
U(1) phase transition.Comment: 3 pages, uuencoded Z-compressed .tar file, to appear in proceedings
of lattice 9
Monopoles in Compact U(1) -- Anatomy of the Phase Transition
We present evidence that the existence of a first order phase transition in
compact U(1) with Wilson action is not related to monopole loops wrapping
around the toroidal lattice, as has been previously suggested. Our analysis is
based on the suppression of such loops by `soft boundary conditions' that
correspond to an infinitely large chemical potential for the monopoles on the
boundary, during the updating process. It is observed that the double peak
structure characteristic for the first order phase transition reappears at
sufficiently large lattice sizes and separations from the lattice boundary.Comment: 8 pages, (color) ps-figures available via anonymous ftp at
ftp://wpts0.physik.uni-wuppertal.de/pub/monopoles/figures.u
The eta ' signal from partially quenched Wilson fermions
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 figure
Cost of QCD simulations with n_f=2 dynamical Wilson fermions
Cost estimates for simulations of full QCD with n_f=2 Wilson fermions by
hybrid Monte Carlo are presented. The extrapolations are based on the average
number of iterations of the iterative solver within the fermionic part of the
HMC molecular dynamics, which is closely related to the minimal eigenvalue of
M^+M The cost formula is determined as a product of the scaling functions of
iterative solver and integrated autocorrelation time as function of the inverse
lattice pseudoscalar mass. Timings by SESAM/TxL allow to fix the pre-factor. It
is demonstrated that a 2-flavor dynamical determination of light hadron masses
with a statistical precision comparable to the corresponding quenched results
from CP-PACS can be an appropriate task for a 100 Tflops system.Comment: Contribution to Lattice2001 (panel discussion), 2 pages, 2 figures,
eq 1 correcte
Accelerating Wilson Fermion Matrix Inversions by Means of the Stabilized Biconjugate Gradient Algorithm
The stabilized biconjugate gradient algorithm BiCGStab recently presented by
van der Vorst is applied to the inversion of the lattice fermion operator in
the Wilson formulation of lattice Quantum Chromodynamics. Its computational
efficiency is tested in a comparative study against the conjugate gradient and
minimal residual methods. Both for quenched gauge configurations at beta= 6.0
and gauge configurations with dynamical fermions at beta=5.4, we find BiCGStab
to be superior to the other methods. BiCGStab turns out to be particularly
useful in the chiral regime of small quark masses.Comment: 25 pages, WUB 94-1
- …