286 research outputs found
Topological tunneling with Dynamical overlap fermions
Tunneling between different topological sectors with dynamical chiral
fermions is difficult because of a poor mass scaling of the pseudo-fermion
estimate of the determinant. For small fermion masses it is virtually
impossible using standard methods. However, by projecting out the small Wilson
eigenvectors from the overlap operator, and treating the correction determinant
exactly, we can significantly increase the rate of topological sector tunneling
and reduce substantially the auto-correlation time. We present and compare a
number of different approaches, and advocate a method which allows topological
tunneling even at low mass with little addition to the computational cost.Comment: 17 pages; v2 as accepted in computer Physics Communication
Nucleon form factors with light Wilson quarks
We present nucleon observables - primarily isovector vector form factors -
from calculations using 2+1 flavors of Wilson quarks. One ensemble is used for
a dedicated high-precision study of excited-state effects using five
source-sink separations between 0.7 and 1.6 fm. We also present results from a
larger set of calculations that include an ensemble with pion mass 149 MeV and
box size 5.6 fm, which nearly eliminates the uncertainty associated with
extrapolation to the physical pion mass. The results show agreement with
experiment for the vector form factors, which occurs only when excited-state
contributions are reduced. Finally, we show results from a subset of ensembles
that have pion mass 254 MeV with varying temporal and spatial box sizes, which
we use for a controlled study of finite-volume effects and a test of the
"" rule of thumb.Comment: 7 pages, 3 figures. Talk presented at the 31st International
Symposium on Lattice Field Theory (Lattice 2013), July 29-August 3, 2013,
Mainz, German
Adaptive Aggregation Based Domain Decomposition Multigrid for the Lattice Wilson Dirac Operator
In lattice QCD computations a substantial amount of work is spent in solving
discretized versions of the Dirac equation. Conventional Krylov solvers show
critical slowing down for large system sizes and physically interesting
parameter regions. We present a domain decomposition adaptive algebraic
multigrid method used as a precondtioner to solve the "clover improved" Wilson
discretization of the Dirac equation. This approach combines and improves two
approaches, namely domain decomposition and adaptive algebraic multigrid, that
have been used seperately in lattice QCD before. We show in extensive numerical
test conducted with a parallel production code implementation that considerable
speed-up over conventional Krylov subspace methods, domain decomposition
methods and other hierarchical approaches for realistic system sizes can be
achieved.Comment: Additional comparison to method of arXiv:1011.2775 and to
mixed-precision odd-even preconditioned BiCGStab. Results of numerical
experiments changed slightly due to more systematic use of odd-even
preconditionin
Towards Power Characterization of FPGA Architectures To Enable Open-Source Power Estimation Using Micro-Benchmarks
While in the past decade there has been significant progress in open-source
synthesis and verification tools and flows, one piece is still missing in the
open-source design automation ecosystem: a tool to estimate the power
consumption of a design on specific target technologies. We discuss a
work-in-progress method to characterize target technologies using generic
micro-benchmarks, whose results can be used to establish power models of these
target technologies. These models can further be used to predict the power
consumption of a design in a given use case scenario (which is currently out of
scope). We demonstrate our characterization method on the publicly documented
Lattice iCE40 FPGA technology, and discuss two approaches to generating
micro-benchmarks which consume power in the target device: simple lookup table
(LUT) instantiation, and a more sophisticated instantiation of ring
oscillators. We study three approaches to stimulate the implemented
micro-benchmarks in hardware: Verilog testbenches, micro-controller
testbenches, and pseudo-random linear-feedback-shift-register-(LFSR)-based
testing. We measure the power consumption of the stimulated target devices. Our
ultimate goal is to automate power measurements for technology
characterization; Currently, we manually measure the consumed power at three
shunt resistors using an oscilloscope. Preliminary results indicate that we are
able to induce variable power consumption in target devices; However, the
sensitivity of the power characterization is still too low to build expressive
power estimation models.Comment: Presented at the 3rd Workshop on Open-Source Design Automation
(OSDA), 2023 (arXiv:2303.18024
Aggregation-based Multilevel Methods for Lattice QCD
In Lattice QCD computations a substantial amount of work is spent in solving
the Dirac equation. In the recent past it has been observed that conventional
Krylov solvers tend to critically slow down for large lattices and small quark
masses. We present a Schwarz alternating procedure (SAP) multilevel method as a
solver for the Clover improved Wilson discretization of the Dirac equation.
This approach combines two components (SAP and algebraic multigrid) that have
separately been used in lattice QCD before. In combination with a bootstrap
setup procedure we show that considerable speed-up over conventional Krylov
subspace methods for realistic configurations can be achieved.Comment: Talk presented at the XXIX International Symposium on Lattice Field
Theory, July 10-16, 2011, Lake Tahoe, Californi
Computing the nucleon charge and axial radii directly at in lattice QCD
We describe a procedure for extracting momentum derivatives of nucleon matrix
elements on the lattice directly at . This is based on the Rome method
for computing momentum derivatives of quark propagators. We apply this
procedure to extract the nucleon isovector magnetic moment and charge radius as
well as the isovector induced pseudoscalar form factor at and the axial
radius. For comparison, we also determine these quantities with the traditional
approach of computing the corresponding form factors, i.e. and
for the case of the vector current and and
for the axial current, at multiple values followed by
-expansion fits. We perform our calculations at the physical pion mass using
a 2HEX-smeared Wilson-clover action. To control the effects of excited-state
contamination, the calculations were done at three source-sink separations and
the summation method was used. The derivative method produces results
consistent with those from the traditional approach but with larger statistical
uncertainties especially for the isovector charge and axial radii.Comment: 16 pages, 7 figure
Nucleon structure with pion mass down to 149 MeV
We present isovector nucleon observables: the axial, tensor, and scalar
charges and the Dirac radius. Using the BMW clover-improved Wilson action and
pion masses as low as 149 MeV, we achieve good control over chiral
extrapolation to the physical point. Our analysis is done using three different
source-sink separations in order to identify excited-state effects, and we make
use of the summation method to reduce their size.Comment: 7 pages, 5 figures. Talk presented at the 30th International
Symposium on Lattice Field Theory (Lattice 2012), June 24-29, 2012, Cairns,
Australi
High-precision calculation of the strange nucleon electromagnetic form factors
We report a direct lattice QCD calculation of the strange nucleon
electromagnetic form factors and in the kinematic range . For the first time, both and
are shown to be nonzero with high significance. This work uses
closer-to-physical lattice parameters than previous calculations, and achieves
an unprecedented statistical precision by implementing a recently proposed
variance reduction technique called hierarchical probing. We perform
model-independent fits of the form factor shapes using the -expansion and
determine the strange electric and magnetic radii and magnetic moment. We
compare our results to parity-violating electron-proton scattering data and to
other theoretical studies.Comment: 6 pages, 5 figures. v2: references adde
Extracting the Single-Particle Gap in Carbon Nanotubes with Lattice Quantum Monte Carlo
We show how lattice Quantum Monte Carlo simulations can be used to calculate
electronic properties of carbon nanotubes in the presence of strong
electron-electron correlations. We employ the path integral formalism and use
methods developed within the lattice QCD community for our numerical work and
compare our results to empirical data of the Anti-Ferromagnetic Mott Insulating
gap in large diameter tubes.Comment: 8 pages, 5 figures, Lat2017 proceedin
Continuum EoS for QCD with Nf=2+1 flavors
We report on a continuum extrapolated result [arXiv:1309.5258] for the
equation of state (EoS) of QCD with dynamical quark flavors. In this
study, all systematics are controlled, quark masses are set to their physical
values, and the continuum limit is taken using at least three lattice spacings
corresponding to temporal extents up to . A Symanzik improved gauge and
stout-link improved staggered fermion action is used. Our results are available
online [ancillary file to arXiv:1309.5258].Comment: Conference proceedings, 7 pages, 4 figures. Talk presented at 31st
International Symposium on Lattice Field Theory (LATTICE 2013), July 29 -
August 3, 2013, Mainz, German
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