Calorons, instantons and constituent monopoles in SU(3) lattice gauge theory

We analyze the zero-modes of the Dirac operator in quenched SU(3) gauge configurations at non-zero temperature and compare periodic and anti-periodic temporal boundary conditions for the fermions. It is demonstrated that for the different boundary conditions often the modes are localized at different space-time points and have different sizes. Our observations are consistent with patterns expected for Kraan - van Baal solutions of the classical Yang-Mills equations. These solutions consist of constituent monopoles and the zero-modes are localized on different constituents for different boundary conditions. Our findings indicate that the excitations of the QCD vacuum are more structured than simple instanton-like lumps.Comment: Remarks added. To appear in Phys. Rev.

A lattice calculation of the pion form factor with Ginsparg-Wilson-type fermions

Results for Monte Carlo calculations of the electromagnetic vector and scalar form factors of the pion in a quenched simulation are presented. We work with two different lattice volumes up to a spatial size of 2.4 fm at a lattice spacing of 0.148 fm. The pion form factors in the space-like region are determined for pion masses down to 340 MeV.Comment: REVTeX 4, 8 pages, 9 figures, 4 tables; final versio

Quantitative comparison of filtering methods in lattice QCD

We systematically compare filtering methods used to extract topological excitations (like instantons, calorons, monopoles and vortices) from lattice gauge configurations, namely APE-smearing and spectral decompositions based on lattice Dirac and Laplace operators. Each of these techniques introduces ambiguities, which can invalidate the interpretation of the results. We show, however, that all these methods, when handled with care, reveal very similar topological structures. Hence, these common structures are free of ambiguities and faithfully represent infrared degrees of freedom in the QCD vacuum. As an application we discuss an interesting power-law for the clusters of filtered topological charge.Comment: 6 pages, 18 plots in 5 figures; final version as published in EPJ A; section 4 was adde

Testing the self-duality of topological lumps in SU(3) lattice gauge theory

We discuss a simple formula which connects the field-strength tensor to a spectral sum over certain quadratic forms of the eigenvectors of the lattice Dirac operator. We analyze these terms for the near zero-modes and find that they give rise to contributions which are essentially either self-dual or anti self-dual. Modes with larger eigenvalues in the bulk of the spectrum are more dominated by quantum fluctuations and are less (anti) self-dual. In the high temperature phase of QCD we find considerably reduced (anti) self-duality for the modes near the edge of the spectral gap.Comment: Remarks added, to appear in Phys. Rev. Let

Search for the Theta^+(1540) in lattice QCD

We report on a study of the pentaquark Theta^+(1540), using a variety of different interpolating fields. We use Chirally Improved fermions in combination with Jacobi smeared quark sources to improve the signal and get reliable results even for small quark masses. The results of our quenched calculations, which have been done on a 12^3 x 24 lattice with a lattice spacing of a = 0.148 fm, do not provide any evidence for the existence of a \Theta^+ with positive parity. We do observe, however, a signal compatible with nucleon-kaon scattering state. For the negative parity the results are inconclusive, due to the potential mixture with nucleon-kaon and N^*-kaon scattering states.Comment: 5 pages, 2 figure

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

Chiral symmetry restoration and the Z3 sectors of QCD

Quenched SU(3) lattice gauge theory shows three phase transitions, namely the chiral, the deconfinement and the Z3 phase transition. Knowing whether or not the chiral and the deconfinement phase transition occur at the same temperature for all Z3 sectors could be crucial to understand the underlying microscopic dynamics. We use the existence of a gap in the Dirac spectrum as an order parameter for the restoration of chiral symmetry. We find that the spectral gap opens up at the same critical temperature in all Z3 sectors in contrast to earlier claims in the literature.Comment: 4 pages, 4 figure

Preliminary results of the heavy-light meson spectrum using chirally improved light quarks

Using a ``wall'' of quark point sources, we invert the chirally improved Dirac operator to create an ``incoherent'' collection of quark propagators which originate from all spatial points of the source time slice. The lowest-order NRQCD approximation is used to create heavy-quark propagators from the same wall source. However, since the numerical cost involved in computing such heavy-quark propagators is low, we are able to use a number of source gauge paths to establish coherence between the heavy and light quarks at several spatial separations. The resulting collection of heavy-light meson correlators is analyzed to extract the corresponding mass spectrum.Comment: 3 pages, 1 figure, Lattice2004(spectrum), minor corrections adde