837 research outputs found
N* Masses from an Anisotropic Lattice QCD Action
We report N* masses in the spin 3/2 sector from a highly-improved anisotropic
action. States with both positive and negative parity are isolated via a parity
projection method. The extent to which spin projection is needed is examined.
The gross features of the splittings from the nucleon ground state show a trend
consistent with experimental results at the quark masses explored.Comment: Lattice2001(spectrum), 3 pages, 4 figures, new interpolating fiel
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
Over-Improved Stout-Link Smearing
A new over-improved stout-link smearing algorithm, designed to stabilise
instanton-like objects, is presented. A method for quantifying the selection of
the over-improvement parameter, , is demonstrated. The new smearing
algorithm is compared with the original stout-link smearing, and Symanzik
improved smearing through calculations of the topological charge and
visualisations of the topological charge density.Comment: 9 pages, 18 figures, submitted to Physical Review
Singlet baryons in the graded symmetry approach to partially quenched QCD
Progress in the calculation of the electromagnetic properties of baryon
excitations in lattice QCD is presenting new challenges in the determination of
sea-quark loop contributions to matrix elements. A reliable estimation of the
sea-quark loop contributions presents a pressing issue in the accurate
comparison of lattice QCD results with experiment. In this article, an
extension of the graded symmetry approach to partially quenched QCD is
presented, which builds on previous theory by explicitly including
flavor-singlet baryons in its construction. The formalism takes into account
the interactions among both octet and singlet baryons, octet mesons, and their
ghost counterparts; the latter enables the isolation of the quark-flow
disconnected sea-quark loop contributions. The introduction of the
flavor-singlet states anticipates the application of the method to baryon
excitations such as the lowest-lying odd-parity Lambda baryon, the
Lambda(1405), which is considered in detail as a worked example.Comment: arXiv copy updated to published version: Phys. Rev. D 94, 094004
(2016
Chiral Analysis of Quenched Baryon Masses
We extend to quenched QCD an earlier investigation of the chiral structure of
the masses of the nucleon and the delta in lattice simulations of full QCD.
Even after including the meson-loop self-energies which give rise to the
leading and next-to-leading non-analytic behaviour (and hence the most rapid
variation in the region of light quark mass), we find surprisingly little
curvature in the quenched case. Replacing these meson-loop self-energies by the
corresponding terms in full QCD yields a remarkable level of agreement with the
results of the full QCD simulations. This comparison leads to a very good
understanding of the origins of the mass splitting between these baryons.Comment: 23 pages, 6 figure
Power Counting Regime of Chiral Effective Field Theory and Beyond
Chiral effective field theory complements numerical simulations of quantum
chromodynamics (QCD) on a space-time lattice. It provides a model-independent
formalism for connecting lattice simulation results at finite volume and a
variety of quark masses to the physical world. The asymptotic nature of the
chiral expansion places the focus on the first few terms of the expansion.
Thus, knowledge of the power-counting regime (PCR) of chiral effective field
theory, where higher-order terms of the expansion may be regarded as
negligible, is as important as knowledge of the expansion itself. Through the
consideration of a variety of renormalization schemes and associated
parameters, techniques to identify the PCR where results are independent of the
renormalization scheme are established. The nucleon mass is considered as a
benchmark for illustrating this general approach. Because the PCR is small, the
numerical simulation results are also examined to search for the possible
presence of an intrinsic scale which may be used in a nonperturbative manner to
describe lattice simulation results outside of the PCR. Positive results that
improve on the current optimistic application of chiral perturbation theory
beyond the PCR are reported.Comment: 18 pages, 55 figure
Chiral extrapolations for nucleon magnetic moments
Lattice QCD simulations have made significant progress in the calculation of
nucleon electromagnetic form factors in the chiral regime in recent years. With
simulation results achieving pion masses of order ~180 MeV, there is an
apparent challenge as to how the physical regime is approached. By using
contemporary methods in chiral effective field theory, both the quark-mass and
finite-volume dependence of the isovector nucleon magnetic moment are carefully
examined. The extrapolation to the physical point yields a result that is
compatible with experiment, albeit with a combined statistical and systematic
uncertainty of 10%. The extrapolation shows a strong finite-volume dependence;
lattice sizes of L > 5 fm must be used to simulate results within 2% of the
infinite-volume result for the magnetic moment at the physical pion mass.Comment: 7 pages, 12 figures, 1 tabl
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