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