893 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
Strangeness contributions to nucleon form factors
We review a recent theoretical determination of the strange quark content of
the electromagnetic form factors of the nucleon. These are compared with a
global analysis of current experimental measurements in parity-violating
electron scattering.Comment: 5 pages, 6 figures; Talk presented at the International Workshop
"From Parity Violation to Hadronic Structure and more...", Milos, Greece, May
16-20, 200
Chiral extrapolation of nucleon magnetic form factors
The extrapolation of nucleon magnetic form factors calculated within lattice
QCD is investigated within a framework based upon heavy baryon chiral
effective-field theory. All one-loop graphs are considered at arbitrary
momentum transfer and all octet and decuplet baryons are included in the
intermediate states. Finite range regularisation is applied to improve the
convergence in the quark-mass expansion. At each value of the momentum transfer
(), a separate extrapolation to the physical pion mass is carried out as a
function of alone. Because of the large values of involved, the
role of the pion form factor in the standard pion-loop integrals is also
investigated. The resulting values of the form factors at the physical pion
mass are compared with experimental data as a function of and demonstrate
the utility and accuracy of the chiral extrapolation methods presented herein.Comment: 19 pages, 10 figure
Lattice QCD Calculations of Hadron Structure: Constituent Quarks and Chiral Symmetry
New data from parity-violating experiments on the deuteron now allow
isolation of the strange-quark contribution to the nucleon magnetic moment,
G_M^s(0), without the uncertainty surrounding the anapole moment of the
nucleon. Still, best estimates place G_M^s(0) > 0. It is illustrated how this
experimental result challenges the very cornerstone of the constituent quark
model. The chiral physics giving rise to G_M^s(0) \sim 0 is illustrated.Comment: Invited talk presented by DBL at the 16th Int. Conf. on Few Body
Problems (Taipei, March 6-10, 2000); 9 pages, 5 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
Chiral behavior of baryon magnetic moments
The utility of chiral effective field theory, constructed in a manner in
which loop contributions are suppressed as one moves outside the power-counting
regime, is explored for baryon magnetic moments. Opportunities for the study of
significant chiral curvature in valence and full QCD and the nontrivial
behavior of strange- and light-quark contributions to the magnetic moment of
the Lambda baryon are highlighted.Comment: 7 pages, 5 figures; prepared for the proceedings of Achievements and
New Directions in Subatomic Physics: Workshop in Honour of Tony Thomas' 60th
Birthda
Chiral extrapolation and physical insights
It has recently been established that finite-range regularisation in chiral
effective field theory enables the accurate extrapolation of modern lattice QCD
results to the chiral regime. We review some of the highlights of
extrapolations of quenched lattice QCD results, including spectroscopy and
magnetic moments. The resonance displays peculiar chiral features in
the quenched theory which can be exploited to demonstrate the presence of
significant chiral corrections.Comment: 6 pages, 5 figures, presented at LHP2003, Cairns, Australi
Hadron structure on the back of an envelope
In order to remove a little of the mysticism surrounding the issue of
strangeness in the nucleon, we present simple, physically transparent estimates
of both the strange magnetic moment and charge radius of the proton. Although
simple, the estimates are in quite good agreement with sophisticated
calculations using the latest input from lattice QCD. We further explore the
possible size of systematic uncertainties associated with charge symmetry
violation (CSV) in the recent precise determination of the strange magnetic
moment of the proton. We find that CSV acts to increase the error estimate by
0.003 \mu_N such that G_M^s = -0.046 +/- 0.022 \mu_N.Comment: 9 pages, 1 figure, Invited talk at First Workshop on Quark-Hadron
Duality and the Transition to pQCD, Frascati, June 6-8 200
Extrapolation of lattice QCD results beyond the power-counting regime
Resummation of the chiral expansion is necessary to make accurate contact
with current lattice simulation results of full QCD. Resummation techniques
including relativistic formulations of chiral effective field theory and
finite-range regularization (FRR) techniques are reviewed, with an emphasis on
using lattice simulation results to constrain the parameters of the chiral
expansion. We illustrate how the chiral extrapolation problem has been solved
and use FRR techniques to identify the power-counting regime (PCR) of chiral
perturbation theory. To fourth-order in the expansion at the 1% tolerance
level, we find 0 \le m_pi \le 0.18 GeV for the PCR, extending only a small
distance beyond the physical pion mass.Comment: 12 pages, 5 figures, plenary talk at BARYONS 2004, Paris, Oct. 25-2
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