Search

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 overimproved 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 203 x 40 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-instantoninteraction predictions of the instanton liquid model. [ABSTRACT FROM AUTHOR]

A recent lattice study of the Landau-gauge overlap quark propagator has shown a connection between center vortices and dynamical chiral symmetry breaking in SU(3) gauge theory. We further investigate this relationship through an exploration of the connection to the instanton degrees of freedom. After identifying center vortices on the lattice in maximal center gauge, we smooth configurations using multiple algorithms. We are able to create an instanton liquid-like background on configurations consisting solely of center vortices, analogous to that found on Monte-Carlo generated configurations after similar smoothing. Through calculations of the static quark potential and Landau-gauge overlap propagator, we show that this background is able to reproduce all salient long-range features of the original configurations. Thus we conclude that the information necessary to recreate the long-range structure of SU(3) gauge theory is contained within the center vortex degrees of freedom. [ABSTRACT FROM AUTHOR]

In 2 + 1 flavor lattice QCD, the spectrum of the nucleon is presented for both parities using local meson-baryon-type interpolating fields in addition to the standard three-quark nucleon interpolators. The role of local five-quark operators in extracting the nucleon excited-state spectrum via correlation matrix techniques is explored on dynamical gauge fields with mπ = 293 MeV, leading to the observation of a state in the region of the noninteracting S-wave Nπ scattering threshold in the negative-parity sector. Furthermore, the robustness of the variational technique is examined by studying the spectrum on a variety of operator bases. Fitting a single-state ansatz to the eigenstate-projected correlators provides robust energies for the low-lying spectrum that are essentially invariant despite being extracted from qualitatively different bases. [ABSTRACT FROM AUTHOR]

We apply the eigenvectors from a variational analysis to successfully extract the three-quark colorsinglet wave functions of even-parity excited states of the nucleón. We explore the first four states in the spectrum excited by the standard nucleón interpolating field. We find that the states exhibit a structure qualitatively consistent with a constituent quark model, where the ground, first, second, and third excited states have 0, 1, 2, and 3 nodes in the radial wave function of the d quark about two u quarks at the origin. Moreover, the radial amplitude of the probability distribution is similar to that predicted by constituent quark models. We present a detailed examination of the quark-mass dependence of the probability distributions for these states, searching for a nontrivial role for the multiparticle components mixed in the finite-volume QCD eigenstates. Finally we examine the dependence of the d-quark probability distribution on the positions of the two u quarks. The results are fascinating, with the underiying 5-wave orbitals governing the distributions even at rather large ti-quark separations. [ABSTRACT FROM AUTHOR]

We present results for the magnetic moment and magnetic polarizability of the neutron and the magnetic moment of the proton. These results are calculated using the uniform background field method on 32³×64 dynamical QCD lattices provided by the PACS-CS Collaboration as part of the ILDG. We use a uniform background magnetic field quantized by the periodic spatial volume. We investigate ways to improve the effective energy plots used to calculate magnetic polarizabilities, including the use of correlation matrix techniques with various source smearings. [ABSTRACT FROM AUTHOR]

The odd-parity nucleon spectrum is examined in the light quark-mass regime in 2 + 1 flavor lattice QCD. Configurations generated by the PACS-CS Collaboration and made available through the International Lattice Data Grid are used, with the lightest pion mass at 156 MeV. A novel method for tracking the individual energy eigenstates as the quark mass changes is introduced. The success of this approach reveals the flow of the states towards the physical masses. Using the correlation-matrix method, the two lowest-energy states revealed are found to be in accord with the physical spectrum of nature. [ABSTRACT FROM AUTHOR]

Two measures are defined to evaluate the coupling strength of smeared interpolating operators to hadronic states at a variety of momenta. Of particular interest is the extent to which strong overlap can be obtained with individual high-momentum states. This is vital to exploring hadronic structure at high-momentum transfers on the lattice and addressing interesting phenomena observed experimentally. We consider a novel idea of altering the shape of the smeared operator to match the Lorentz contraction of the probability distribution of the high-momentum state and show a reduction in the relative error of the two-point function by employing this technique. Our most important finding is that the overlap of the states becomes very sharp in the smearing parameters at high momenta, and fine tuning is required to ensure strong overlap with these states. [ABSTRACT FROM AUTHOR]

The impact of SU(3) center vortices on the gluon propagator in Landau gauge is investigated on original, vortex-removed and vortex-only lattice gauge field configurations. Vortex identification is found to partition the gluon propagator into short-range strength on the vortex-removed configurations and long-range strength on the vortex-only configurations. The effect of smoothing vortex-only configurations is also studied, and a regime for recovering the form of the smoothed original propagator from vortex-only configurations is introduced. The results reinforce the significance of center vortices in a fundamental understanding of QCD vacuum structure. [ABSTRACT FROM AUTHOR]

The positive-parity nucleon spectrum is explored in 2+1 flavor lattice QCD in a search for new low-lying energy eigenstates near the energy regime of the Roper resonance. In addition to conventional three-quark operators, we consider novel, local five-quark meson-baryon type interpolating fields that hold the promise to reveal new eigenstates that may have been missed in previous analyses. Drawing on phenomenological insight, five-quark operators based on σN,pπ N and a0N channels are constructed. Spectra are produced in a high-statistics analysis on the PACS-CS dynamical gauge-field configurations with mπ=411 MeV via variational analyses of several operator combinations. Despite the introduction of qualitatively different interpolating fields, no new states are observed in the energy regime of the Roper resonance. This result provides further evidence that the low-lying finite-volume scattering states are not localized, and strengthens the interpretation of the Roper as a coupled-channel, dynamically-generated meson-baryon resonance. [ABSTRACT FROM AUTHOR]

In a recent study of the Λ(1405) the suppression of the strange-quark contribution to the magnetic form factor was interpreted as the discovery of a dominant antikaon-nucleon composition for this low-lying state. We confirm this result by calculating the light u and d quark contributions to the Λ(1405) magnetic form factor in lattice QCD in order to determine the extent to which their contributions support this exotic molecular description. Drawing on the recent graded-symmetry approach for the flavor-singlet components of the Λ(1405), the separation of connected and disconnected contributions is performed in both the flavor-octet and -singlet representations. In both cases, the disconnected-loop contributions are found to be large. The relationship between light-quark contributions to the Λ(1405) magnetic form factor and the connected contributions of the nucleon magnetic form factors is established and compared with lattice calculations of the same quantities, confirming the KN molecular structure of the Λ(1405) in lattice QCD. [ABSTRACT FROM AUTHOR]

We examine the phase shifts and inelasticities associated with the N*(1440) Roper resonance, and we connect these infinite-volume observables to the finite-volume spectrum of lattice QCD using Hamiltonian effective field theory. We explore three hypotheses for the structure of the Roper resonance. All three hypotheses are able to describe the scattering data well. In the third hypothesis the Roper resonance couples the low-lying bare basis-state component associated with the ground-state nucleon with the virtual meson-baryon contributions. Here the nontrivial superpositions of the meson-baryon scattering states are complemented by bare basis-state components, explaining their observation in contemporary lattice QCD calculations. The merit of this scenario lies in its ability to not only describe the observed nucleon energy levels in large-volume lattice QCD simulations but also explain why other low-lying states have been missed in today's lattice QCD results for the nucleon spectrum. [ABSTRACT FROM AUTHOR]

We calculate the probability distributions of quarks in the ground state of the proton, and how they are affected in the presence of a constant background magnetic field. We focus on wave functions in the Landau and Coulomb gauges. We observe the formation of a scalar u-d diquark clustering. The overall distortion of the quark probability distribution under a very large magnetic field, as demanded by the quantization conditions on the field, is quite small. The effect is to elongate the distributions along the external field axis while localizing the remainder of the distribution. [ABSTRACT FROM AUTHOR]

The application of a uniform background magnetic field makes standard quark operators utilizing gauge-covariant Gaussian smearing inefficient at isolating the ground state nucleon at nontrivial field strengths. In the absence of QCD interactions, Landau modes govern the quark energy levels. There is evidence that residual Landau mode effects remain when the strong interaction is turned on. Here, we introduce novel quark operators constructed from the two-dimensional U(1) Laplacian eigenmodes that describe the Landau levels of a charged particle on a periodic finite lattice. These eigenmode-projected quark operators provide enhanced precision for calculating nucleon energy shifts in a magnetic field. Using asymmetric source and sink operators, we are able to encapsulate the predominant effects of both the QCD and QED interactions in the interpolating fields for the neutron. The neutron magnetic polarizability is calculated using these techniques on the 32³×64 dynamical QCD lattices provided by the PACS-CS Collaboration. In conjunction with a chiral effective-field theory analysis, we obtain a neutron magnetic polarizability of ßn=2.05(25)(19)×10-4 fm³, where the numbers in parentheses describe statistical and systematic uncertainties. [ABSTRACT FROM AUTHOR]