605 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

### 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
($Q^2$), a separate extrapolation to the physical pion mass is carried out as a
function of $m_\pi$ alone. Because of the large values of $Q^2$ 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 $Q^2$ and demonstrate
the utility and accuracy of the chiral extrapolation methods presented herein.Comment: 19 pages, 10 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

### 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

### 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

### QCD Equalities for Baryon Current Matrix Elements

An examination of the symmetries manifest in the QCD path integral for
current matrix elements reveals various equalities among the quark sector
contributions. QCD equalities among octet baryon magnetic moments lead to a
determination of the disconnected sea-quark contribution to nucleon magnetic
moments, which is the most reliable determination in the literature. Matching
QCD equalities to recent calculations of decuplet baryon magnetic moments in
chiral perturbation theory (ChiPT) reveals an equivalence between ChiPT to
order $p^2$ and the simple quark model with an explicit disconnected sea-quark
contribution. New insights into SU(3)-flavor symmetry breaking, sea
contributions and constituent quark composition are obtained. The strangeness
contribution to nucleon magnetic moments is found to be large at $G_M^s(0) =
-0.75 \pm 0.30\ \mu_N$. The QCD equalities must be followed by any model which
hopes to capture the essence of nonperturbative QCD. Not all models are in
accord with these symmetries.Comment: 20 page RevTeX Manuscript with embedded figures. This and related
papers may also be obtained from
http://www.phys.washington.edu/~derek/Publications.htm

### 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 $\Delta$ 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

### Physical Baryon Resonance Spectroscopy from Lattice QCD

We complement recent advances in the calculation of the masses of excited
baryons in quenched lattice QCD with finite-range regulated chiral effective
field theory enabling contact with the physical quark mass region. We examine
the P-wave contributions to the low-lying nucleon and delta resonances.Comment: Contributed paper at FB17, the 17th International Conference on
Few-Body Problems in Physics, Durham, NC, June 5-10, 2003. 3 pages, 6 figure

### 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

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