arXiv subjects

D. B. Leinweber

Publications and source records attributed to D. B. Leinweber.

At least 91 records · Page 5Linked to original sources

arXiv2003

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 $Δ$ resonance displays peculiar chiral features in the quenched theory which can be exploited to demonstrate the presence of significant chiral corrections.

hep-lat↗

arXiv2003

Electromagnetic Form Factors with FLIC Fermions

The Fat-Link Irrelevant Clover (FLIC) fermion action provides a new form of nonperturbative O(a) improvement and allows efficient access to the light quark-mass regime. FLIC fermions enable the construction of the nonperturbatively O(a)-improved conserved vector current without the difficulties associated with the fine tuning of the improvement coefficients. The simulations are performed with an O(a^2) mean-field improved plaquette-plus-rectangle gluon action on a 20^3 x 40 lattice with a lattice spacing of 0.128 fm, enabling the first simulation of baryon form factors at light quark masses on a large volume lattice. Magnetic moments, electric charge radii and magnetic radii are extracted from these form factors, and show interesting chiral nonanalytic behavior in the light quark mass regime.

hep-lat↗

arXiv2003

Chiral Structure in Baryon Magnetic Moments

We highlight recent advances in finite-range regularised chiral effective field theory. Application to baryon magnetic moments show an unambiguous signal of quenched chiral physics in FLIC fermion simulations.

hep-lat↗

arXiv2003

Dynamical FLIC Fermions

The use of APE smearing or other blocking techniques in fermion actions can provide many advantages. There are many variants of these fat link actions in lattice QCD currently, such as FLIC fermions. The FLIC fermion formalism makes use of the APE blocking technique in combination with a projection of the blocked links back into the special unitary group. This reunitarisation is often performed using an iterative maximisation of a gauge invariant measure. This technique is not differentiable with respect to the gauge field and thus prevents the use of standard Hybrid Monte Carlo simulation algorithms. The use of an alternative projection technique circumvents this difficulty and allows the simulation of dynamical fat link fermions with standard HMC and its variants.

hep-lat↗

arXiv2003

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.

nucl-th↗

arXiv2003

Observing Chiral Nonanalytic Behavior with FLIC Fermions

First results from lattice QCD revealing the chiral nonanalytic behavior of nucleon and Delta baryon magnetic moments are presented. Numerical simulations in the light quark mass regime employing the nonperturbatively O(a)-improved conserved vector current are enabled via FLIC fermions. Quenched chiral perturbation theory for the nucleon and Delta magnetic moments is derived to next to next to leading nonanalytic order. Numerical simulation results for the proton and Delta baryon magnetic moments in quenched QCD reveal dramatic signatures of the quenched meson cloud, which are in accord with the predictions of quenched chiral perturbation theory.

nucl-th↗

arXiv2003

Nucleon electromagnetic form factors from lattice QCD

It is imperative that lattice QCD serve to develop our understanding of hadron structure and, where possible, to guide the interpretation of experimental data. There is now a great deal of effort directed at the calculation of the electroweak form factors of the nucleon, where for example, measurements at Jefferson Laboratory have recently revealed surprising behaviour in the ratio $G_E/G_M$. While, for the present, calculations within the framework of lattice QCD are limited to relatively large quark mass, there has been considerable progress in our understanding of how to extrapolate to the chiral limit. Here we report the results of the application of these techniques to the most recent form factor data from the QCDSF Collaboration. The level of agreement with all of the form factors, for $Q^2$ below 1 GeV$^2$, is already impressive.

hep-lat↗

arXiv2003

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.

hep-lat↗

arXiv2003

Delta Baryon Magnetic Moments From Lattice QCD

Theoretical predictions for the magnetic moments of the physical Delta baryons are extracted from lattice QCD calculations. We utilize finite-range regulated effective field theory that is constructed to have the correct Dirac moment mass dependence in the region where the up and down quark masses are heavy. Of particular interest is the chiral nonanalytic behaviour encountered as the nucleon-pion decay channel opens. We find a Delta^++ magnetic moment (at the Delta pole) of 4.99 \pm 0.56 μ_N. This result is within the Particle Data Group range of 3.7-7.5 μ_N and compares well with the experimental result of Bosshard et al. of 4.52 \pm 0.51 \pm 0.45 μ_N. The interplay between the different pion-loop contributions to the Delta^+ magnetic moment leads to the surprising result that the proton moment may exceed that of the Delta^+, contrary to conventional expectations.

hep-lat↗

arXiv2003

Nonperturbative structure of the quark-gluon vertex

The complete tensor structure of the quark--gluon vertex in Landau gauge is determined at two kinematical points (`asymmetric' and `symmetric') from lattice QCD in the quenched approximation. The simulations are carried out at beta=6.0, using a mean-field improved Sheikholeslami-Wohlert fermion action, with two quark masses ~ 60 and 115 MeV. We find substantial deviations from the abelian form at the asymmetric point. The mass dependence is found to be negligible. At the symmetric point, the form factor related to the chromomagnetic moment is determined and found to contribute significantly to the infrared interaction strength.

hep-ph↗

arXiv2003

Excited Baryons in Lattice QCD

We present first results for the masses of positive and negative parity excited baryons calculated in lattice QCD using an O(a^2)-improved gluon action and a fat-link irrelevant clover (FLIC) fermion action in which only the irrelevant operators are constructed with APE-smeared links. The results are in agreement with earlier calculations of N^* resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner. An correlation matrix analysis reveals two low-lying J^P=(1/2)^- states with a small mass splitting. The study of different Lambda interpolating fields suggests a similar splitting between the lowest two Lambda1/2^- octet states. However, the empirical mass suppression of the Lambda^*(1405) is not evident in these quenched QCD simulations, suggesting a potentially important role for the meson cloud of the Lambda^*(1405) and/or a need for more exotic interpolating fields.

hep-lat↗

arXiv2003

Physical Nucleon Properties from Lattice QCD

We demonstrate that the extremely accurate lattice QCD data for the mass of the nucleon recently obtained by CP-PACS, combined with modern chiral extrapolation techniques, leads to a value for the mass of the physical nucleon which has a systematic error of less than one percent.

hep-lat↗

arXiv2002

Convergence of chiral effective field theory

We formulate the expansion for the mass of the nucleon as a function of pion mass within chiral perturbation theory using a number of different ultra-violet regularisation schemes; including dimensional regularisation and various finite-ranged regulators. Leading and next-to-leading order non-analytic contributions are included through the standard one-loop Feynman graphs. In addition to the physical nucleon mass, the expansion is constrained by recent, extremely accurate, lattice QCD data obtained with two flavors of dynamical quarks. The extent to which different regulators can describe the chiral expansion is examined, while varying the range of quark mass over which the expansions are matched. Renormalised chiral expansion parameters are recovered from each regularisation prescription and compared. We find that the finite-range regulators produce consistent, model-independent results over a wide range of quark mass sufficient to solve the chiral extrapolation problem in lattice QCD.

hep-lat↗

arXiv2002

Baryon Resonance Phenomenology

The Japan Hadron Facility will provide an unprecedented opportunity for the study of baryon resonance properties. This talk will focus on the chiral nonanalytic behaviour of magnetic moments exclusive to baryons with open decay channels. To illustrate the novel features associated with an open decay channel, we consider the ``Access'' quark model, where an analytic continuation of chiral perturbation theory is employed to connect results obtained using the constituent quark model in the limit of SU(3)-flavour symmetry to empirical determinations.

nucl-th↗

arXiv2002

Quenched Chiral Physics in Baryon Masses

Recent work has identified that the primary differences between quenched and dynamical spectroscopy can be described by chiral loop effects. Here we highlight the features of this study.

nucl-th↗

arXiv2002

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.

nucl-th↗

arXiv2002

Light Quark Simulations With FLIC Fermions

Hadron masses are calculated in quenched lattice QCD in order to probe the scaling behavior of a novel fat-link clover fermion action in which only the irrelevant operators of the fermion action are constructed using APE-smeared links. Light quark masses corresponding to an m_pi / m_rho ratio of 0.35 are considered to assess the exceptional configuration problem of clover-fermion actions. This Fat-Link Irrelevant Clover (FLIC) fermion action provides scaling which is superior to mean-field improvement and offers advantages over nonperturbative improvement, including reduced exceptional configurations.

hep-lat↗

arXiv2002

Excited Baryons from the FLIC Fermion Action

Masses of positive and negative parity excited nucleons and hyperons are calculated in quenched lattice QCD using an O(a^2) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier N* simulations with improved actions, and exhibit a clear mass splitting between the nucleon and its parity partner, as well as a small mass splitting between the two low-lying J^P={1/2}^- N* states. Study of different Lambda interpolating fields suggests a similar splitting between the lowest two {1/2}^- Lambda* states, although the empirical mass suppression of the Lambda*(1405) is not seen.

hep-lat↗