720 research outputs found
A Lattice QCD Analysis of the Strangeness Magnetic Moment of the Nucleon
The outcome of the SAMPLE Experiment suggests that the strange-quark
contribution to the nucleon magnetic moment, G_M^s(0), may be greater than
zero. This result is very difficult to reconcile with expectations based on the
successful baryon magnetic-moment phenomenology of the constituent quark model.
We show that careful consideration of chiral symmetry reveals some rather
unexpected properties of QCD. In particular, it is found that the valence
u-quark contribution to the magnetic moment of the neutron can differ by more
than 50% from its contribution to the Xi^0 magnetic moment. This hitherto
unforeseen result leads to the value G_M^s(0) = -0.16 +/- 0.18 with a
systematic error, arising from the relatively large strange quark mass used in
existing lattice calculations, that would tend to shift G_M^s(0) towards small
positive values.Comment: RevTeX, 20 pages, 12 figure
Testing QCD Sum Rule Techniques on the Lattice
Results for the first test of the ``crude'' QCD continuum model, commonly
used in QCD Sum Rule analyses, are presented for baryon correlation functions.
The QCD continuum model is found to effectively account for excited state
contributions to the short-time regime of two-point correlation functions and
allows the isolation of ground state properties. Confusion in the literature
surrounding the physics represented in point-to-point correlation functions is
also addressed. These results justify the use of the ``crude'' QCD continuum
model and lend credence to the results of rigorous QCD Sum Rule analyses.Comment: Discussion of systematic uncertainties augmente
Bosonic stringlike behavior and the Ultraviolet filtering of QCD
The gluonic action density is calculated in static mesons at finite
temperature just below the deconfinement point. Our focus is to elucidate the
role of vacuum ultraviolet fluctuations which are filtered using an improved
smearing algorithm. In the intermediate source separation distance, where the
free string picture poorly describes the flux tube width profile, we find upon
reducing the vacuum action towards the classical instanton vacuum, the
characteristics of the flux tube converge and compare favorably with the
predictions of the free bosonic string. This result establishes a connection
between the free string action and vacuum gauge fields and reveals the
important role of ultraviolet physics in understanding the lattice data at this
temperature scale. As a by-product of these calculations, we find the
broadening of the QCD flux tube to be independent of the ultraviolet filtering
at large distances. Our results exhibit a linearly divergent pattern in
agreement with the string picture predictions.Comment: 8 pages, 8 Figures, 1 Tabl
Highly-improved lattice field-strength tensor
We derive an O(a^4)-improved lattice version of the continuum field-strength
tensor. Discretization errors are reduced via the combination of several clover
terms of various sizes, complemented by tadpole improvement. The resulting
improved field-strength tensor is used to construct O(a^4)-improved topological
charge and action operators. We compare the values attained by these operators
as we cool several configurations to self-duality with a previously defined
highly-improved action and assess the relative scale of the remaining
discretization errors.Comment: 22 pages, 7 postscript figure
Accelerated Overlap Fermions
Numerical evaluation of the overlap Dirac operator is difficult since it
contains the sign function of the Hermitian Wilson-Dirac
operator with a negative mass term. The problems are due to having
very small eigenvalues on the equilibrium background configurations generated
in current day Monte Carlo simulations. Since these are a consequence of the
lattice discretisation and do not occur in the continuum version of the
operator, we investigate in this paper to what extent the numerical evaluation
of the overlap can be accelerated by making the Wilson-Dirac operator more
continuum-like. Specifically, we study the effect of including the clover term
in the Wilson-Dirac operator and smearing the link variables in the irrelevant
terms. In doing so, we have obtained a factor of two speedup by moving from the
Wilson action to a FLIC (Fat Link Irrelevant Clover) action as the overlap
kernel.Comment: 15 pages, 6 figures; V2 contains major revision of the introduction
and motivation sections. Conclusion and results unchanged v2.1: formatting
chang
Searching for low-lying multi-particle thresholds in lattice spectroscopy
We explore the Euclidean-time tails of odd-parity nucleon correlation
functions in a search for the S-wave pion-nucleon scattering-state threshold
contribution. The analysis is performed using 2+1 flavor 32^3 x 64 PACS-CS
gauge configurations available via the ILDG. Correlation matrices composed with
various levels of fermion source/sink smearing are used to project low-lying
states. The consideration of 25,600 fermion propagators reveals the presence of
more than one state in what would normally be regarded as an
eigenstate-projected correlation function. This observation is in accord with
the scenario where the eigenstates contain a strong mixing of single and
multi-particle states but only the single particle component has a strong
coupling to the interpolating field. Employing a two-exponential fit to the
eigenvector-projected correlation function, we are able to confirm the presence
of two eigenstates. The lower-lying eigenstate is consistent with a N-pi
scattering threshold and has a relatively small coupling to the three-quark
interpolating field. We discuss the impact of this small scattering-state
contamination in the eigenvector projected correlation function on previous
results presented in the literature.Comment: 8 pages, 4 figures. Manuscript accepted for publicatio
FLIC Overlap Fermions
The action of the overlap-Dirac operator on a vector is typically implemented
indirectly through a multi-shift conjugate gradient solver. The compute-time
required depends upon the condition number, , of the matrix that is
used as the overlap kernel. While the Wilson action is typically used as the
overlap kernel, the FLIC (Fat Link Irrelevant Clover) action has an improved
condition number and provides up to a factor of two speedup in evaluating the
overlap action. We summarize recent progress on the use of FLIC overlap
fermions.Comment: Lattice2002(chiral
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