185 research outputs found
Algorithms in Lattice QCD
The enormous computing resources that large-scale simulations in Lattice QCD
require will continue to test the limits of even the largest supercomputers into
the foreseeable future. The efficiency of such simulations will therefore concern
practitioners of lattice QCD for some time to come.
I begin with an introduction to those aspects of lattice QCD essential to the
remainder of the thesis, and follow with a description of the Wilson fermion
matrix M, an object which is central to my theme.
The principal bottleneck in Lattice QCD simulations is the solution of linear
systems involving M, and this topic is treated in depth. I compare some of the
more popular iterative methods, including Minimal Residual, Corij ugate Gradient
on the Normal Equation, BI-Conjugate Gradient, QMR., BiCGSTAB and
BiCGSTAB2, and then turn to a study of block algorithms, a special class of iterative
solvers for systems with multiple right-hand sides. Included in this study
are two block algorithms which had not previously been applied to lattice QCD.
The next chapters are concerned with a generalised Hybrid Monte Carlo algorithm
(OHM C) for QCD simulations involving dynamical quarks. I focus squarely
on the efficient and robust implementation of GHMC, and describe some tricks
to improve its performance. A limited set of results from HMC simulations at
various parameter values is presented.
A treatment of the non-hermitian Lanczos method and its application to the
eigenvalue problem for M rounds off the theme of large-scale matrix computations
Tuning Actions and Observables in Lattice QCD
We propose a strategy for conducting lattice QCD simulations at fixed volume
but variable quark mass so as to investigate the physical effects of dynamical
fermions. We present details of techniques which enable this to be carried out
effectively, namely the tuning in bare parameter space and efficient stochastic
estimation of the fermion determinant. Preliminary results and tests of the
method are presented. We discuss further possible applications of these
techniques.Comment: 17 pages, 4 eps figures; affiliation correction in this header +
minor post-referee addition
Infrared identification of IGR J09026-4812 as a Seyfert 1 galaxy
IGR J09026-4812 was discovered by INTEGRAL in 2006 as a new hard X-ray
source. Thereafter, an observation with Chandra pinpointed a single X-ray
source within the ISGRI error circle, showing a hard spectrum, and improving
its high-energy localisation to a subarcsecond accuracy. Thus, the X-ray source
was associated to the infrared counterpart 2MASS J09023731-4813339 whose JHKs
photometry indicated a highly reddened source. The high-energy properties and
the counterpart photometry suggested a high-mass X-ray binary with a main
sequence companion star located 6.3-8.1 kpc away and with a 0.3-10 keV
luminosity of 8e34 erg/s. New optical and infrared observations were needed to
confirm the counterpart and to reveal the nature of IGR J09026-4812. We
performed optical and near infrared observations on the counterpart 2MASS
J09023731-4813339 with the ESO/NTT telescope on March 2007. We achieved
photometry and spectroscopy in near infrared wavelengths and photometry in
optical wavelengths. The accurate astrometry at both optical and near infrared
wavelengths confirmed 2MASS J09023731-4813339 to be the counterpart of IGR
J09026-4812. However, the near infrared images show that the source is
extended, thus excluding any Galactic compact source possibility. The source
spectrum shows three main emission lines identified as the HeI lambda 1.0830
micron line, and the HI Pa_beta and Pa_alpha lines, typical in galaxies with an
active galactic nucleus. The broadness of these lines reached values as large
as 4000 km/s pointing towards a type 1 Seyfert galaxy. The redshift of the
source is z=0.0391(4). Thus, the near infrared photometry and spectroscopy
allowed us to classify IGR J09026-4812 as a Seyfert galaxy of type 1.Comment: 4 pages, 3 figures, Astronomy and Astrophysics in pres
Instability in the Molecular Dynamics Step of Hybrid Monte Carlo in Dynamical Fermion Lattice QCD Simulations
We investigate instability and reversibility within Hybrid Monte Carlo simulations using a non-perturbatively improved Wilson action. We demonstrate the onset of instability as tolerance parameters and molecular dynamics step sizes are varied. We compare these findings with theoretical expectations and present limits on simulation parameters within which a stable and reversible algorithm is obtained for physically relevant simulations. Results of optimisation experiments with respect to tolerance prarameters are also presented
Instability in the molecular dynamics step of a hybrid Monte Carlo algorithm in dynamical fermion lattice QCD simulations
We investigate instability and reversibility within Hybrid Monte Carlo
simulations using a non-perturbatively improved Wilson action. We demonstrate
the onset of instability as tolerance parameters and molecular dynamics step
sizes are varied. We compare these findings with theoretical expectations and
present limits on simulation parameters within which a stable and reversible
algorithm is obtained for physically relevant simulations. Results of
optimisation experiments with respect to tolerance prarameters are also
presented.Comment: RevTeX, Some results here were presented at Vielat 99, Vienna,
Austria, Sept 1999 22 Pages, 10 figures, to be submitted to PR
Chelator free gallium-68 radiolabelling of silica coated iron oxide nanorods via surface interactions
The commercial availability of combined magnetic resonance imaging (MRI)/positron emission tomography (PET) scanners for clinical use has increased demand for easily prepared agents which offer signal or contrast in both modalities. Herein we describe a new class of silica coated ironâoxide nanorods (NRs) coated with polyethylene glycol (PEG) and/or a tetraazamacrocyclic chelator (DO3A). Studies of the coated NRs validate their composition and confirm their properties as in vivo Tâ MRI contrast agents. Radiolabelling studies with the positron emitting radioisotope gallium-68 (t1/2 = 68 min) demonstrate that, in the presence of the silica coating, the macrocyclic chelator was not required for preparation of highly stable radiometal-NR constructs. In vivo PET-CT and MR imaging studies show the expected high liver uptake of gallium-68 radiolabelled nanorods with no significant release of gallium-68 metal ions, validating our innovation to provide a novel simple method for labelling of iron oxide NRs with a radiometal in the absence of a chelating unit that can be used for high sensitivity liver imaging
Parallel Tempering in Lattice QCD with O(a)-Improved Wilson Fermions
We present details of our investigation of the Parallel Tempering algorithm. We consider the application of action matching technology to the selection of parameters. We then present a simple model of the autocorrelations for a particular parallel tempered system. Finally we present numerical results from applying the algorithm to lattice QCD with O(a)-improved dynamical fermions with twin ensemble system
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