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Robert G. Edwards

Publications and source records attributed to Robert G. Edwards.

At least 73 records · Page 4Linked to original sources
arXiv2010

Nucleon, Delta and Omega excited state spectra at three pion mass values

The energies of the excited states of the Nucleon, Delta and Omega are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculations are performed at three values of the pion mass: 392(4), 438(3) and 521(3) MeV. We employ the variational method with a basis of about ten interpolating operators enabling six energies to be distinguished clearly in each irreducible representation of the octahedral group. We compare our calculations of nucleon excited states with the low-lying experimental spectrum. There is reasonable agreement for the pattern of states.

hep-lat↗
arXiv2010

Nucleon structure from mixed action calculations using 2+1 flavors of asqtad sea and domain wall valence fermions

We present high statistics results for the structure of the nucleon from a mixed-action calculation using 2+1 flavors of asqtad sea and domain wall valence fermions. We perform extrapolations of our data based on different chiral effective field theory schemes and compare our results with available information from phenomenology. We discuss vector and axial form factors of the nucleon, moments of generalized parton distributions, including moments of forward parton distributions, and implications for the decomposition of the nucleon spin.

hep-lat↗
arXiv2010

Lattice Calculations of Nucleon Electromagnetic Form Factors at Large Momentum Transfer

In this work, we report a novel technique in lattice QCD for studying the high momentum-transfer region of nucleon form factors. These calculations could give important theoretical input to experiments, such as those of JLab's 12-GeV program and studies of nucleon deformation. There is an extensive history of form-factor calculations on the lattice, primarily with ground states for both the initial and final state. However, determining form factors at large momentum transfer ($Q^2$) has been difficult due to large statistical and systematic errors in this regime. We study the nucleon form factors using three pion masses with both quenched and 2+1-flavor anisotropic lattice configurations with $Q^2$ as large as $6 GeV^2$. These form factors are further processed to obtain transverse charge and magnetization densities across 2-dimensional impact-parameter space. Our approach can be applied to isotropic lattices and lattices with smaller lattice spacing to calculate even larger-$Q^2$ form factors.

hep-lat↗
arXiv2010

Toward the excited meson spectrum of dynamical QCD

We present a detailed description of the extraction of the highly excited isovector meson spectrum on dynamical anisotropic lattices using a new quark-field construction algorithm and a large variational basis of operators. With careful operator construction, the combination of these techniques is used to identify the continuum spin of extracted states reliably, overcoming the reduced rotational symmetry of the cubic lattice. Excited states, states with exotic quantum numbers (0+-, 1-+ and 2+-) and states of high spin are resolved, including, for the first time in a lattice QCD calculation, spin-four states. The determinations of the spectrum of isovector mesons and kaons are performed on dynamical lattices with two volumes and with pion masses down to ~ 400 MeV, with statistical precision typically at or below 1% even for highly excited states.

hep-ph↗
arXiv2009

Excited-Nucleon Spectroscopy with 2+1 Fermion Flavors

We present progress made by the Hadron Spectrum Collaboration (HSC) in determining the tower of excited nucleon states using 2+1-flavor anisotropic clover lattices. The HSC has been investigating interpolating operators projected into irreducible representations of the cubic group in order to better calculate two-point correlators for nucleon spectroscopy; results are published for quenched and 2-flavor anisotropic Wilson lattices. In this work, we present the latest results using a new technique, distillation, which allows us to reach higher statistics than before. Future directions will be outlined at the end.

hep-lat↗
arXiv2009

Highly excited and exotic meson spectrum from dynamical lattice QCD

Using a new quark-field construction algorithm and a large variational basis of operators, we extract a highly excited isovector meson spectrum on dynamical anisotropic lattices. We show how carefully constructed operators can be used to reliably identify the continuum spin of extracted states, overcoming the reduced cubic symmetry of the lattice. Using this method we extract, with confidence, excited states, states with exotic quantum numbers (0+-, 1-+ and 2+-) and states of high spin, including for the first time in lattice QCD, spin-four states.

hep-ph↗
arXiv2009

An anisotropic preconditioning for the Wilson fermion matrix on the lattice

A preconditioning for the Wilson fermion matrix on the lattice is defined which is particularly suited to the case when the temporal lattice spacing is much smaller than the spatial one. Details on the implementation of the scheme are given. The method is tested in numerical studies of QCD on anisotropic lattices.

hep-lat↗
arXiv2009

A novel quark-field creation operator construction for hadronic physics in lattice QCD

A new quark-field smearing algorithm is defined which enables efficient calculations of a broad range of hadron correlation functions. The technique applies a low-rank operator to define smooth fields that are to be used in hadron creation operators. The resulting space of smooth fields is small enough that all elements of the reduced quark propagator can be computed exactly at reasonable computational cost. Correlations between arbitrary sources, including multi hadron operators can be computed a posteriori without requiring new lattice Dirac operator inversions. The method is tested on realistic lattice sizes with light dynamical quarks.

hep-lat↗
arXiv2008

Excited State Nucleon Spectrum with Two Flavors of Dynamical Fermions

Highly excited states for isospin 1/2 baryons are calculated for the first time using lattice QCD with two flavors of dynamical quarks. Anisotropic lattices are used with two pion masses: 416(36) MeV and 578(29) MeV. The lowest four energies are reported in each of the six irreducible representations of the octahedral group at each pion mass. The lattices used have dimensions 24^3x64, spatial lattice spacing a_s = 0.11 fm and temporal lattice spacing a_t = 1/3 a_s. Clear evidence is found for a 5/2^-state in the pattern of negative-parity excited states. This agrees with the pattern of physical states and spin 5/2 has been realized for the first time on the lattice.

hep-lat↗
arXiv2008

Cascade Baryon Spectrum from Lattice QCD

A comprehensive study of the cascade baryon spectrum using lattice QCD affords the prospect of predicting the masses of states not yet discovered experimentally, and determining the spin and parity of those states for which the quantum numbers are not yet known. The study of the cascades, containing two strange quarks, is particularly attractive for lattice QCD in that the chiral effects are reduced compared to states composed only of u/d quarks, and the states are typically narrow. We report preliminary results for the cascade spectrum obtained by using anisotropic N_f = 2 Wilson lattices with temporal lattice spacing inverse 5.56 GeV.

hep-lat↗
arXiv2008

Light-Meson Two-Photon Decays in Full QCD

We present a study of two-photon decays of light mesons, focusing on the neutral pion decay. This important process highlights the effects of the axial anomaly in QCD but has been little studied on the lattice. By applying the Lehmann-Symanzik-Zimmermann (LSZ) reduction formula, we reconstruct the electromagnetic matrix elements from three-point vector-vector Green functions calculated on 2+1-flavor isotropic clover lattices.

hep-lat↗
arXiv2008

Challenges in Hadronic Form Factor Calculations

There is an extensive history of form factor calculations on the lattice, primarily with ground states for both initial and final states. However, there have never been any radially excited transition form factor calculations. Furthermore, the lattice faces difficulty in extracting signal from noise at large transfer momenta ($Q^2$). These measurements could give important theoretical input to experiments, such as those of JLab's 12 GeV program and studies of deformation of the nucleon. In this work, I will present a simple technique to resolve both of these difficulties and present results from anisotropic configurations showing improved signals for excited-state quantities. It should also be possible to apply this technique to isotropic lattices for calculating large-$Q^2$ form factors.

hep-lat↗
arXiv2008

First results from 2+1 dynamical quark flavors on an anisotropic lattice: light-hadron spectroscopy and setting the strange-quark mass

We present the first light-hadron spectroscopy on a set of $N_f=2+1$ dynamical, anisotropic lattices. A convenient set of coordinates that parameterize the two-dimensional plane of light and strange-quark masses is introduced. These coordinates are used to extrapolate data obtained at the simulated values of the quark masses to the physical light and strange-quark point. A measurement of the Sommer scale on these ensembles is made, and the performance of the hybrid Monte Carlo algorithm used for generating the ensembles is estimated.

hep-lat↗
arXiv2008

Excited nucleon spectrum with two flavors of dynamical fermions

We compute the spectrum of excited nucleons using the anisotropic Wilson lattice with two flavors of dynamical fermions. Using optimized sets of operators which transform irreducibly under the octahedral group, matrices of correlation functions are computed. We apply the variational method to these matrices to extract excited energy eigenstates. We obtain several states for each irrep and identify the continuum spin for the lowest-lying states, including a $J^P={5/2}^-$ state.

hep-lat↗
arXiv2008

Tuning for Three-flavors of Anisotropic Clover Fermions with Stout-link Smearing

In this work we perform the parameter tuning of three flavors of dynamical clover quarks on anisotropic lattices. The fermion action uses three-dimensional spatial stout-link smearing. The gauge anisotropy is determined in a small box with Schrödinger background using Wilson-loop ratios. The fermion anisotropy is obtained from studying the meson dispersion relation with antiperiodic boundary conditions in the time direction. The spatial and temporal clover coefficients are fixed to the tree-level tadpole-improved values, and we demonstrate that they satisfy the nonperturbative conditions as determined by the Schrödinger functional method. For the desired lattice spacing $a_s\approx 0.12$ fm and renormalized anisotropy $ξ=3.5$, we find the gauge and fermionic anisotropies can be fixed to quark mass independent values up through the strange quark mass. This work lays the foundation needed for further studies of the excited-state hadron spectrum.

hep-lat↗
arXiv2008

First Lattice Study of the $N$-$P_{11}(1440)$ Transition Form Factors

Experiments at Jefferson Laboratory, MIT-Bates, LEGS, Mainz, Bonn, GRAAL, and Spring-8 offer new opportunities to understand in detail how nucleon resonance ($N^*$) properties emerge from the nonperturbative aspects of QCD. Preliminary data from CLAS collaboration, which cover a large range of photon virtuality $Q^2$ show interesting behavior with respect to $Q^2$ dependence: in the region $Q^2 \le 1.5 {GeV}^2$, both the transverse amplitude, $A_{1/2}(Q^2)$, and the longitudinal amplitude, $S_{1/2}(Q^2)$, decrease rapidly. In this work, we attempt to use first-principles lattice QCD (for the first time) to provide a model-independent study of the Roper-nucleon transition form factor.

hep-lat↗
arXiv2007

Parameter Tuning of Three-Flavor Dynamical Anisotropic Clover Action

In this work, we perform parameter tuning with dynamical anisotropic clover lattices using the Schrödinger functional and stout-smearing in the fermion field. We find that $ξ_R/ξ_0$ is relatively close to 1 in our parameter search, which allows us to fix $ξ_0$ in our runs. We proposed to determine the gauge and fermion anisotropy in a Schrödinger-background small box using Wilson loop ratios and PCAC masses. We demonstrate that these ideas are equivalent to but more efficient than the conventional meson dispersion approach. The spatial and temporal clover coefficients are fixed to the tree-level tadpole-improved clover values, and we demonstrate that they satisfy the nonperturbative condition determined by Schrödinger functional method.

hep-lat↗