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

Infrared Behavior of the Gluon Propagator on a Large Volume Lattice

The first calculation of the gluon propagator using an order a^2 improved action with the corresponding order a^2 improved Landau gauge fixing condition is presented. The gluon propagator obtained from the improved action and improved Landau gauge condition is compared with earlier unimproved results on similar physical lattice volumes of 3.2^3 \times 6.4 fm^4. We find agreement between the improved propagator calculated on a coarse lattice with lattice spacing a = 0.35 fm and the unimproved propagator calculated on a fine lattice with spacing a = 0.10 fm. This motivates us to calculate the gluon propagator on a coarse large-volume lattice 5.6^3 \times 11.2 fm^4. The infrared behavior of previous studies is confirmed in this work. The gluon propagator is enhanced at intermediate momenta and suppressed at infrared momenta. Therefore the observed infrared suppression of the Landau gauge gluon propagator is not a finite volume effect.Comment: 8 pages, 4 figures, minor typos corrected and repsonse to referees comment

Chiral extrapolation of lattice data for B-meson decay constant

The B-meson decay constant fB has been calculated from unquenched lattice QCD in the unphysical region. For extrapolating the lattice data to the physical region, we propose a phenomenological functional form based on the effective chiral perturbation theory for heavy mesons, which respects both the heavy quark symmetry and the chiral symmetry, and the non-relativistic constituent quark model which is valid at large pion masses. The inclusion of pion loop corrections leads to nonanalytic contributions to fB when the pion mass is small. The finite-range regularization technique is employed for the resummation of higher order terms of the chiral expansion. We also take into account the finite volume effects in lattice simulations. The dependence on the parameters and other uncertainties in our model are discussed.Comment: 11 pages, 3 Postscript figures, accepted for publication in EPJ

FLIC Fermions and Hadron Phenomenology

A pedagogical overview of the formulation of the Fat Link Irrelevant Clover (FLIC) fermion action and its associated phenomenology is described. The scaling analysis indicates FLIC fermions provide a new form of nonperturbative O(a) improvement where near-continuum results are obtained at finite lattice spacing. Spin-1/2 and spin-3/2, even and odd parity baryon resonances are investigated in quenched QCD, where the nature of the Roper resonance and Lambda(1405) are of particular interest. FLIC fermions allow efficient access to the light quark-mass regime, where evidence of chiral nonanalytic behavior in the Delta-baryon mass is observed.Comment: Invited plenary session talk at QNP 2002, International Conference on Quark-Nuclear Physics, 9-14 June 2002, Forschungszentrum Julich, German

Simple quark model with chiral phenomenology

We propose a new approach to the determination of hadronic observables in which the essential features of chiral symmetry are combined with conventional constituent quark models. To illustrate the approach, we consider the simple quark model in the limit of SU~3! flavor symmetry at the strange quark mass. The comparison with data is made after an analytic continuation which ensures the correct leading nonanalytic behavior of chiral perturbation theory. The approach not only gives an excellent fit for the octet baryon magnetic moments but the prediction for the D11 magnetic moment is also in good agreement with current measurements.I. C. Cloet, D. B. Leinweber, and A. W. Thoma

Chiral Extrapolation of Lattice Data for Heavy Baryons

The masses of heavy baryons containing a b quark have been calculated numerically in lattice QCD with pion masses which are much larger than its physical value. In the present work we extrapolate these lattice data to the physical mass of the pion by applying the effective chiral Lagrangian for heavy baryons, which is invariant under chiral symmetry when the light quark masses go to zero and heavy quark symmetry when the heavy quark masses go to infinity. A phenomenological functional form with three parameters, which has the correct behavior in the chiral limit and appropriate behavior when the pion mass is large, is proposed to extrapolate the lattice data. It is found that the extrapolation deviates noticably from the naive linear extrapolation when the pion mass is smaller than about 500MeV. The mass differences between Sigma_b and Sigma_b^* and between Sigma_b^{(*)} and Lambda_b are also presented. Uncertainties arising from both lattice data and our model parameters are discussed in detail. We also give a comparision of the results in our model with those obtained in the naive linear extrapolations.Comment: 29 pages, 9 figure

Light-Quark FLIC Fermion Simulations of the 1−+1^{-+} Exotic Meson

We investigate the mass of the $1^{-+}$ exotic meson, created with hybrid interpolating fields. Access to light quark masses approaching 25 MeV is facilitated by the use of the Fat-Link Irrelevant Clover (FLIC) fermion action, and large ($20^3 \times 40$) lattices. Our results indicate that the $1^{-+}$ exotic exhibits significant curvature close to the chiral limit, and yield a $1^{-+}$ mass in agreement with the $\pi_1 (1600)$ candidate and exclusive of the $\pi_1 (1400)$.Comment: 6 pages, 1 table, 2 figures, talk given at Lattice '05. Removed unccessary figure

Spin-3/2 Nucleon and Delta Baryons in Lattice QCD

We present first results for masses of spin-3/2 N and Delta baryons in lattice QCD using Fat-Link Irrelevant Clover (FLIC) fermions. Spin-3/2 interpolating fields providing overlap with both spin-3/2 and spin-1/2 states are considered. In the isospin-1/2 sector, we observe, after appropriate spin and parity projection, a strong signal for the J^P=3/2^- state together with a weak but discernible signal for the 3/2^+ state with a mass splitting near that observed experimentally. We also find good agreement between the 1/2^+/- masses and earlier nucleon mass simulations with the standard spin-1/2 interpolating field. For the isospin-3/2 Delta states, clear mass splittings are observed between the various 1/2^+/- and 3/2^+/- channels, with the calculated level orderings in good agreement with those observed empirically.Comment: 17 pages, 8 figures, 2 table

Baryon Spectroscopy in Lattice QCD

We review recent developments in the study of excited baryon spectroscopy in lattice QCD. After introducing the basic methods used to extract masses from correlation functions, we discuss various interpolating fields and lattice actions commonly used in the literature. We present a survey of results of recent calculations of excited baryons in quenched QCD, and outline possible future directions in the study of baryon spectra.Comment: Contribution to Lecture Notes in Physics on Lattice Hadron Physics, 43 pages, 11 figures, 3 table