219 research outputs found
Decorrelation of the topological charge in tempered Hybrid Monte Carlo simulations of QCD
We study the improvement of simulations of QCD with dynamical Wilson fermions
by combining the Hybrid Monte Carlo algorithm with parallel tempering. As an
indicator for decorrelation we use the topological charge.Comment: LATTICE99 (algorithms and machines
Calm Multi-Baryon Operators
Outstanding problems in nuclear physics require input and guidance from
lattice QCD calculations of few baryons systems. However, these calculations
suffer from an exponentially bad signal-to-noise problem which has prevented a
controlled extrapolation to the physical point. The variational method has been
applied very successfully to two-meson systems, allowing for the extraction of
the two-meson states very early in Euclidean time through the use of improved
single hadron operators. The sheer numerical cost of using the same techniques
in two-baryon systems has been prohibitive. We present an alternate strategy
which offers some of the same advantages as the variational method while being
significantly less numerically expensive. We first use the Matrix Prony method
to form an optimal linear combination of single baryon interpolating fields
generated from the same source and different sink interpolators. Very early in
Euclidean time this linear combination is numerically free of excited state
contamination, so we coin it a calm baryon. This calm baryon operator is then
used in the construction of the two-baryon correlation functions.
To test this method, we perform calculations on the WM/JLab iso-clover gauge
configurations at the SU(3) flavor symmetric point with m{\pi} 800 MeV
--- the same configurations we have previously used for the calculation of
two-nucleon correlation functions. We observe the calm baryon removes the
excited state contamination from the two-nucleon correlation function to as
early a time as the single-nucleon is improved, provided non-local (displaced
nucleon) sources are used. For the local two-nucleon correlation function
(where both nucleons are created from the same space-time location) there is
still improvement, but there is significant excited state contamination in the
region the single calm baryon displays no excited state contamination.Comment: 8 pages, 3 figures, proceedings for LATTICE 201
Speeding up Lattice QCD simulations with clover-improved Wilson Fermions
We apply a recent proposal to speed up the Hybrid-Monte-Carlo simulation of
systems with dynamical fermions to two flavour QCD with clover-improvement. The
basic idea of our proposal is to split the fermion matrix into two factors with
a reduced condition number each. In the effective action, for both factors a
pseudo-fermion field is introduced. For our smallest quark masses we see a
speed-up of more than a factor of two compared with the standard algorithm.Comment: 26 pages, 2 figure
The nucleon mass in N_f=2 lattice QCD: finite size effects from chiral perturbation theory
In the framework of relativistic SU(2)_f baryon chiral perturbation theory we
calculate the volume dependence of the nucleon mass up to and including O(p^4).
Since the parameters in the resulting finite size formulae are fixed from the
pion mass dependence of the large volume nucleon masses and from phenomenology,
we obtain a parameter-free prediction of the finite size effects. We present
mass data from the recent N_f=2 simulations of the UKQCD and QCDSF
collaborations and compare these data as well as published mass values from the
dynamical simulations of the CP-PACS and JLQCD collaborations with the
theoretical expectations. Remarkable agreement between the lattice data and the
predictions of chiral perturbation theory in a finite volume is found.Comment: 23 pages, 5 figures; references added + minor corrections; one more
reference added, typo in eq.(25) corrected, additional clarifying remark
Lattice QCD with mixed actions
We discuss some of the implications of simulating QCD when the action used
for the sea quarks is different from that used for the valence quarks. We
present exploratory results for the hadron mass spectrum and pseudoscalar meson
decay constants using improved staggered sea quarks and HYP-smeared overlap
valence quarks. We propose a method for matching the valence quark mass to the
sea quark mass and demonstrate it on UKQCD clover data in the simpler case
where the sea and valence actions are the same.Comment: 15 pages, 10 figures some minor modification to text and figures.
Accepted for publicatio
Simulating the weak death of the neutron in a femtoscale universe with near-Exascale computing
The fundamental particle theory called Quantum Chromodynamics (QCD) dictates
everything about protons and neutrons, from their intrinsic properties to
interactions that bind them into atomic nuclei. Quantities that cannot be fully
resolved through experiment, such as the neutron lifetime (whose precise value
is important for the existence of light-atomic elements that make the sun shine
and life possible), may be understood through numerical solutions to QCD. We
directly solve QCD using Lattice Gauge Theory and calculate nuclear observables
such as neutron lifetime. We have developed an improved algorithm that
exponentially decreases the time-to solution and applied it on the new CORAL
supercomputers, Sierra and Summit. We use run-time autotuning to distribute GPU
resources, achieving 20% performance at low node count. We also developed
optimal application mapping through a job manager, which allows CPU and GPU
jobs to be interleaved, yielding 15% of peak performance when deployed across
large fractions of CORAL.Comment: 2018 Gordon Bell Finalist: 9 pages, 9 figures; v2: fixed 2 typos and
appended acknowledgement
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