25,673 research outputs found
Bound states for Overlap and Fixed Point Actions close to the chiral limit
We study the overlap and the fixed point Dirac operators for massive fermions
in the two-flavor lattice Schwinger model. The masses of the triplet (pion) and
singlet (eta) bound states are determined down to small fermion masses and the
mass dependence is compared with various continuum model approximations. Near
the chiral limit, at very small fermion masses the fixed point operator has
stability problems, which in this study are dominated by finite size effects,Comment: 13 pages, 2 figure
Nucleon excited states on the lattice
We study the pion-nucleon system in s-wave in the framework of lattice QCD in
order to gain new information on the nucleon excited states. We perform
simulations for mass degenerate light quarks at a pion mass of 266
MeV. The results show that including the two-particle states drastically
changes the energy levels. The variational analysis and the distillation
approach play an important role in the extraction of the energy levels. The
phase shift analysis allows to extract information on the resonance nature of
the observed states.Comment: 6 pages, 2 figures, talk presented at Excited QCD 2013, Bjelasnica
Mountain, Sarajev
Dirac zero-modes in compact U(1) gauge theory
We study properties of the zero and near-zero eigenmodes of the overlap Dirac
operator in compact U(1) gauge theory. In the confinement phase the exact
zero-modes are localized as found by studying the values of the inverse
participation ratio and other features. Non-zero-eigenmodes are less localized
in the confinement phase. In the Coulomb phase no zero-modes are observed and
the eigenmodes show no localization at all.Comment: Minor corrections, 15 pages, 5 figures, LaTeX styl
Symmetries of hadrons after unbreaking the chiral symmetry
We study hadron correlators upon artificial restoration of the spontaneously
broken chiral symmetry. In a dynamical lattice simulation we remove the lowest
lying eigenmodes of the Dirac operator from the valence quark propagators and
study evolution of the hadron masses obtained. All mesons and baryons in our
study, except for a pion, survive unbreaking the chiral symmetry and their
exponential decay signals become essentially better. From the analysis of the
observed spectroscopic patterns we conclude that confinement still persists
while the chiral symmetry is restored. All hadrons fall into different chiral
multiplets. The broken U(1)_A symmetry does not get restored upon unbreaking
the chiral symmetry. We also observe signals of some higher symmetry that
includes chiral symmetry as a subgroup. Finally, from comparison of the \Delta
- N splitting before and after unbreaking of the chiral symmetry we conclude
that both the color-magnetic and the flavor-spin quark-quark interactions are
of equal importance.Comment: 12 pages, 14 figures; final versio
Topological Charge and the Spectrum of the Fermion Matrix in Lattice-QED_2
We investigate the interplay between topological charge and the spectrum of
the fermion matrix in lattice-QED_2 using analytic methods and Monte Carlo
simulations with dynamical fermions. A new theorem on the spectral
decomposition of the fermion matrix establishes that its real eigenvalues (and
corresponding eigenvectors) play a role similar to the zero eigenvalues (zero
modes) of the Dirac operator in continuous background fields. Using numerical
techniques we concentrate on studying the real part of the spectrum. These
results provide new insights into the behaviour of physical quantities as a
function of the topological charge. In particular we discuss fermion
determinant, effective action and pseudoscalar densities.Comment: 33 pages, 10 eps-figures; reference adde
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