Fat-Link Irrelevant Clover Research Articles

Instanton contributions to the low-lying hadron mass spectrum

The role of instanton-like objects in the QCD vacuum on the mass spectrum of low-lying light hadrons is explored in lattice QCD. Using over-improved stout-link smearing, tuned to preserve instanton-like objects in the QCD vacuum, the evolution of the mass spectrum under smearing is examined. The calculation is performed using a $20^3\times40$ dynamical fat-link-irrelevant-clover (FLIC) fermion action ensemble with lattice spacing 0.126 fm. Through the consideration of a range of pion masses, the effect of the vacuum instanton content is compared at a common pion mass. While the qualitative features of ground-state hadrons are preserved on instanton-dominated configurations, the excitation spectrum experiences significant changes. The underlying physics revealed shows little similarity to the direct-instanton interaction predictions of the instanton liquid model.

Electromagnetic structure of decuplet baryons towards the chiral regime

The electromagnetic properties of the baryon decuplet are calculated in quenched QCD on a 20^3 x 40 lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover (FLIC) fermion action with quark masses providing a pion mass as low as 300 MeV. Magnetic moments and charge radii are extracted from the electric and magnetic form factors for each individual quark sector. From these, the corresponding baryon properties are constructed. We present results for the higher order moments of the spin-3/2 baryons, including the electric quadrupole moment E2 and the magnetic octupole moment M3. The world's first determination of a non-zero M3 form factor for the Delta baryon is presented. With these results we provide a conclusive analysis which shows that decuplet baryons are deformed. We compare the decuplet baryon results from a similar lattice calculation of the octet baryons. We establish that the environment sensitivity is far less pronounced in the case of the decuplet baryons compared to that in the octet baryons. A surprising result is that the charge radii of the decuplet baryons are generally smaller than that of the octet baryons. The magnetic moment of the Delta^+ reveals a turn over in the low quark mass region, making it smaller than the proton magnetic moment. These results are consistent with the expectations of quenched chiral perturbation theory. A similar turn over is also noticed in the magnetic moment of the Sigma^*0, but not for Xi^* where only kaon loops can appear in quenched QCD. The electric quadrupole moment of the Omega^- baryon is positive when the negative charge factor is included, and is equal to 0.86 +- 0.12 x 10^-2 fm^2, indicating an oblate shape.

Scaling analysis of fat-link irrelevant clover fermion actions

The fat-link irrelevant clover fermion action is a variant of the O(a)-improved Wilson action where the irrelevant operators are constructed using smeared links. While the use of such smearing allows for the use of highly improved definitions of the field strength tensor F{sub {mu}}{sub {nu}}, we show that the standard 1-loop clover term with a mean field improved coefficient c{sub sw} is sufficient to remove the O(a) errors, avoiding the need for nonperturbative tuning. This result enables efficient dynamical simulations in QCD with the fat-link irrelevant clover fermion action.

Unquenching effects in the quark and gluon propagator

In this work we examine the fat-link irrelevant clover (FLIC) overlap quark propagator and the gluon propagator on both dynamical and quenched lattices. The tadpole-improved Luscher-Weisz gauge action is used in both cases. The dynamical gauge fields use the FLIC fermion action for the sea quark contribution. We observe that the presence of sea quarks causes a suppression of the mass function, quark renormalization function, and gluon dressing function in the infrared. The ultraviolet physics is unaffected.

Pseudoscalar and vector meson form factors from lattice QCD

We present a study of the pseudoscalar and vector meson form factors, calculated using the fat-link irrelevant clover (FLIC) action in the framework of quenched lattice QCD. Of particular interest is the determination of a negative quadrupole moment, indicating that the $\ensuremath{\rho}$ meson is not spherically symmetric.

Precision electromagnetic structure of octet baryons in the chiral regime

The electromagnetic properties of the baryon octet are calculated in quenched QCD on a ${20}^{3}\ifmmode\times\else\texttimes\fi{}40$ lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover (FLIC) fermion action. FLIC fermions enable simulations to be performed efficiently at pion masses as low as 300 MeV. By combining FLIC fermions with an improved-conserved vector current, we ensure that discretization errors occur only at $\mathcal{O}({a}^{2})$ while maintaining current conservation. Magnetic moments and electric and magnetic radii are extracted from the electric and magnetic form factors for each individual quark sector. From these, the corresponding baryon properties are constructed. Our results are compared with the predictions of quenched chiral perturbation theory. We detect substantial curvature and environment sensitivity of the quark contributions to electric charge radii and magnetic moments in the low quark-mass region.

1−+exotic meson at light quark masses

The mass of the ${1}^{\ensuremath{-}+}$ exotic meson, created with hybrid interpolating fields, is explored in numerical simulations of quenched QCD on large (${20}^{3}\ifmmode\times\else\texttimes\fi{}40$) lattices to obtain good control of statistical and finite volume errors. Using the Fat-Link Irrelevant Clover (FLIC) fermion action, the properties of the ${1}^{\ensuremath{-}+}$ are investigated at light quark masses approaching 25 MeV (${m}_{\ensuremath{\pi}}/{m}_{\ensuremath{\rho}}\ensuremath{\simeq}1/3$). Under the standard assumption that the coupling to the quenched ${a}_{1}{\ensuremath{\eta}}^{\ensuremath{'}}$ channel comes with a negative metric, our results indicate that the ${1}^{\ensuremath{-}+}$ exotic exhibits significant curvature close to the chiral limit, suggesting previous linear extrapolations have overestimated the mass of the ${1}^{\ensuremath{-}+}$. We find for the first time in lattice studies a ${1}^{\ensuremath{-}+}$ mass consistent with the ${\ensuremath{\pi}}_{1}(1600)$ candidate. We also find a strangeness $\ifmmode\pm\else\textpm\fi{}1$ ${J}^{P}={1}^{\ensuremath{-}}$ state with a mass close to 2 GeV.

Fat link irrelevant clover overlap quark propagator

FLIC overlap fermions are a variant of the standard (Wilson) overlap action, with the FLIC (Fat Link Irrelevant Clover) action as the overlap kernel rather than the Wilson action. The structure of the FLIC overlap fermion propagator in momentum space is studied, and a comparison against previous studies of the Wilson overlap propagator in quenched QCD is performed. To explore the scaling properties of the propagator for the two actions, numerical calculations are performed in Landau Gauge across three lattices with different lattice spacing $a$ and similar physical volumes. We find that at light quark masses the acti ons agree in both the infrared and the ultraviolet, but at heavier masses some disagreement in the ultraviolet appears. This is attributed to the two action s having different discretisation errors with the FLIC overlap providing superior performance in this regime. Both actions scale reasonably, but some scaling violations are observed.

Improved chiral properties of FLIC fermions

The chiral properties of the fat-link irrelevant clover (FLIC) fermion action are examined. The improved chiral properties of fermion actions incorporating smoothed links are realised in the FLIC action where only the irrelevant operators of the fermion action are constructed with smoothed links. In particular, the histogram of the additive mass renormalisation encountered in chiral-symmetry breaking Wilson-type fermion actions is seen to narrow upon introducing fat links in the irrelevant operators. The exceptional configuration problem of quenched QCD is reduced, enabling access to the light quark mass regime of mπ/mρ≃1/3. In particular, quenched chiral nonanalytic behaviour is revealed in the light quark mass dependence of the Δ-baryon mass. FLIC fermions offer a promising approach to revealing the properties of full QCD at light quark masses.

Scaling of fat-link irrelevant-clover fermions

Hadron masses are calculated in quenched lattice QCD on a variety of lattices in order to probe the scaling behavior of the Fat-Link Irrelevant Clover (FLIC) fermion action, a fat-link clover fermion action in which the purely irrelevant operators of the fermion action are constructed using APE-smeared links. 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.

Properties of the FLIC overlap quark propagator

FLIC overlap fermions are a type of smeared link action in which the FLIC (Fat Link Irrelevant Clover) fermion action is used as the overlap kernel instead of the standard Wilson fermion action. We study the (quenched) quark propagator in momentum space using the FLIC overlap fermion action, and compare it to that of the Wilson overlap. We find that the two actions have different discretization errors which decrease with the quark mass. We also examine the FLIC overlap quark propagator within the dynamical kernel approximation, where the FLIC Overlap action is used for the valence quarks and the FLIC fermion action is used for the quark sea. Our results indicate that the quark propagator is relatively insensitive to the fermion sea.

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.

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.

Spin-3/2 nucleon andΔbaryons in lattice QCD

We present the first results for masses of spin-$\frac{3}{2}$ N and $\ensuremath{\Delta}$ baryons in lattice QCD using fat-link irrelevant clover fermions. Spin-$\frac{3}{2}$ interpolating fields, providing overlap with both spin-$\frac{3}{2}$ and spin-$\frac{1}{2}$ states, are considered. In the isospin-$\frac{1}{2}$ sector we observe, after appropriate spin and parity projection, a strong signal for the ${J}^{P}={\frac{3}{2}}^{\ensuremath{-}}$ state together with a weak but discernible signal for the ${\frac{3}{2}}^{+}$ state with a mass splitting near that observed experimentally. We also find good agreement between the ${\frac{1}{2}}^{\ifmmode\pm\else\textpm\fi{}}$ masses and earlier nucleon mass simulations with the standard spin-$\frac{1}{2}$ interpolating field. For the isospin-$\frac{3}{2}$ $\ensuremath{\Delta}$ states, clear mass splittings are observed between the various ${\frac{1}{2}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${\frac{3}{2}}^{\ifmmode\pm\else\textpm\fi{}}$ channels, with the calculated level orderings in good agreement with those observed empirically.

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.

FLIC overlap fermions

The action of the overlap-Dirac operator on a vector is typically implemented indirectly through a multi-shift conjugate gradient solver. The compute-time required depends upon the condition number, κ, of the matrix that is used as the overlap kernel. While the Wilson action is typically used as the overlap kernel, the FLIC (Fat Link Irrelevant Clover) action has an improved condition number and provides up to a factor of two speedup in evaluating the overlap action. We summarize recent progress on the use of FLIC overlap fermions.

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 APEsmeared links. Light quark masses providing am π/m p mass 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 non-perturbative improvement including a reduced exceptional configuration problem.

FLIC-overlap fermions and topology

APE smearing the links in the irrelevant operators of clover fermions (Fat-Link Irrelevant Clover (FLIC) fermions) provides significant improvement in the condition number of the Hermitian-Dirac operator and gives rise to a factor of two savings in computing the overlap operator. This report investigates the effects of using a highly-improved definition of the lattice field-strength tensor F μν in the fermion action, made possible through the use of APE-smeared fat links in the construction of the irrelevant operators. Spurious double-zero crossings in the spectral flow of the Hermitian-Wilson Dirac operator associated with lattice artifacts at the scale of the lattice spacing are removed with FLIC fermions composed with anO (α 4 )-improved lattice field strength tensor. Hence, FLIC-Overlap fermions provide an additional benefit to the overlap formalism: a correct realization of topology in the fermion sector on the lattice.

Nucleon Resonances from FLIC Fermions

The Fat Link Irrelevant Clover (FLIC) fermion action and its associated phenomenology are 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 and spin- 3 , even and odd parity nucleon resonances are investigated. §1. FLIC fermions The CSSM lattice collaboration has been examining the merits of a new lattice fermion action 1) in which the (irrelevant) operators introduced to remove fermion doublers and lattice spacing artifacts are constructed with smoothed (APE smeared 2) ) links. The use of links in which short-distance fluctuations have been removed simplifies the determination of the coefficients of the improvement terms. 3) Perturbative renormalizations are small for smeared links and tree-level estimates, or the mean-field improved estimates used here, are sufficient to remove O(a) errors in the lattice spacing a. The key is that both the energy dimension-five Wilson term and the Clover term 4) are constructed with smooth links, while the relevant operators, surviving in the continuum limit, are constructed with the original untouched links generated via standard Monte Carlo techniques. A key feature of FLIC fermions is that the construction of irrelevant operators using smoothed links facilitates the use of highly improved definitions of the QCD field strength tensor Fµν leaving errors of O(a 6 ). 5) We call this action the Fat-Link-Irrelevant-Clover (FLIC) fermion action. Figure 1 displays our most comprehensive scaling analysis to date. The FLIC action performs systematically better than the mean-field improved clover and competes well with those obtained with the NP-improved clover fermion action. 7) There are two different volumes used at a 2 σ ∼ 0.075. These reveal a finite volume effect, which increases the mass for the smaller volumes at a 2 σ ∼ 0.075 and ∼ 0.045. Examination of points from the small and large volumes separately indicates continued scaling toward the continuum limit. Focusing on simulation results from physical volumes with extents ∼ 2 fm and larger, we perform a simultaneous fit of the FLIC, NP-improved clover and Wilson fermion action results. The fits are constrained to have a common continuum limit and assume errors are O(a 2 ) for FLIC and NP-improved clover actions and O(a) for the Wilson action. These results indicate that FLIC fermions provide a new form of

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 ${\mathcal O}(a)$ 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 $\Delta$ mass is observed.