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Andreas Schäfer

Publications and source records attributed to Andreas Schäfer.

At least 181 records · Page 10Linked to original sources
arXiv2006

Excited mesons from $N_f=2$ dynamical Clover Wilson lattices

We study mesons on the lattice with a special focus on excited states. For that purpose we construct several quark sources with different spatial smearings, including p-waves. These quark sources are then combined with the appropiate Dirac structures to form meson interpolators of definite spin. We use these operators to construct a cross correlation matrix from which we extract ground and excited meson states using the variational method. For the calculations we use gauge configurations with $N_f=2$ dynamical Clover Wilson fermions provided by the CP-PACS collaboration. We show preliminary results for pseudoscalar, scalar, vector and pseudovector mesons.

hep-lat↗
arXiv2006

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 Θ^+ 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.

hep-lat↗
arXiv2006

Excited hadrons on the lattice: Mesons

We present results for masses of excited mesons from quenched calculations using chirally improved quarks at pion masses down to 350 MeV. The key features of our analysis are the use of a matrix of correlators from various source and sink operators and a basis which includes quark sources with different spatial widths, thereby improving overlap with states exhibiting radial excitations.

hep-lat↗
arXiv2006

Excited hadrons on the lattice: Baryons

We present results for masses of excited baryons from a quenched calculation with Chirally Improved quarks at pion masses down to 350 MeV. Our analysis of the correlators is based on the variational method. In order to provide a large basis set for spanning the physical states, we use interpolators with different Dirac structures and Jacobi smeared quark sources of different width. Our spectroscopy results for a wide range of ground state and excited baryons are discussed.

hep-lat↗
arXiv2005

Topologically non-trivial field configurations - interplay of vortices and Dirac eigenmodes

Gluon field configurations with non-trivial topology like instantons, magnetic monopoles and center vortices play a crucial role in QCD and, in particular, for the spontaneous breaking of chiral symmetry. Moreover, center vortices are strongly correlated with confinement. We present evidence, that there is a deep connection between the topology of gauge fields and center vortices. We use the chirally improved lattice Dirac operator to compute eigenvectors and eigenvalues of various lattice gauge field configurations. Removing vortices from thermalized configurations also removes the topological content of the gauge field. As a consistency check, we apply random changes to the raw configurations.

hep-lat↗
arXiv2004

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.

hep-lat↗
arXiv2004

Excited hadrons from improved interpolating fields

The calculation of quark propagators for Ginsparg-Wilson-type Dirac operators is costly and thus limited to a few different sources. We present a new approach for determining spatially optimized operators for lattice spectroscopy of excited hadrons. Jacobi smeared quark sources with different widths are combined to construct hadron operators with different spatial wave functions. We study the Roper state and excited rho and pion mesons.

hep-lat↗
arXiv2003

Searching for KvBLL calorons in SU(3) lattice gauge field ensembles

We discuss Kraan - van Baal - Lee - Lu (KvBLL) solutions of the classical Yang-Mills equations with temperature in the context of SU(3) lattice gauge theory. We present discretized lattice versions of KvBLL solutions and other dyonic structures, obtained by cooling in order to understand their variety and signature. An analysis of the zero modes of the lattice Dirac operator for different fermionic boundary conditions gives clear evidence for a KvBLL-like background of finite T lattice subensembles with Q = +/-1. Using APE-smearing we are able to study the topological charge density q(x) of the configurations and to corroborate this interpretation.

hep-lat↗
arXiv2003

Entropy Production in High Energy Processes

We calculate the entropy produced in the decoherence of a classical field configuration and compare it with the entropy of a fully thermalized state with the same energy. We find that decoherence alone accounts for a large fraction of the equilibrium entropy when each field mode is only moderately occupied. We apply this to theories of relativistic heavy ion collisions, which describe the initial state as a collection of coherent color fields. Our results suggest that decoherence may partly explain the rapid formation of a high entropy state in these collisions.

hep-ph↗
arXiv2002

Quenched QCD with fixed-point and chirally improved fermion

In this contribution we present results from quenched QCD simulations with the parameterized fixed-point (FP) and the chirally improved (CI) Dirac operator. Both these operators are approximate solutions of the Ginsparg-Wilson equation and have good chiral properties. We focus our discussion on observables sensitive to chirality. In particular we explore pion masses down to 210 MeV in light hadron spectroscopy, quenched chiral logs, the pion decay constant and the pion scattering length. We discuss finite volume effects, scaling properties of the FP and CI operators and performance issues in their numerical implementation.

hep-lat↗
arXiv2001

A comprehensive picture of topological excitations in finite temperature lattice QCD

We study spectra, localization properties and local chirality of eigenvectors of the lattice Dirac operator. We analyze ensembles of quenched SU(3) configurations on both sides of the QCD phase transition. Our Dirac operator is a systematic expansion in path length of a solution of the Ginsparg-Wilson equation. Analyzing the finite volume behavior of our observables and their scaling with the gauge coupling we come up with a consistent picture of topological excitations on both sides of the QCD phase transition. Our results support the interpretation of the dominant gauge field excitations seen by the Dirac operator as a fluid of instanton-like objects.

hep-lat↗
arXiv2001

Comparing lattice Dirac operators in smooth instanton backgrounds

We compare the behavior of different lattice Dirac operators in gauge backgrounds which are lattice discretizations of a classical instanton. In particular we analyze the standard Wilson operator, a chirally improved Dirac operator and the overlap operators constructed from these two operators. We discuss the flow of real eigenvalues as a function of the instanton size. An analysis of the eigenvectors shows that overlap fermions with the Wilson operator as input operator have difficulties with reproducing the continuum zero mode already for moderately small instantons. This problem is greatly reduced when using the chirally improved operator for the overlap projection.

hep-lat↗
arXiv2001

Chirally improved Dirac operators: Studying the sensitivity to topological excitations for zero and finite temperature

We discuss the construction and properties of an approximate solution of the Ginsparg-Wilson equation, the so-called chirally improved lattice Dirac operator. In particular we study the behavior of its eigenmodes in smooth instanton backgrounds as well as for thermalized gauge configurations on both sides of the QCD phase transition. We compare with results from other Dirac operators including the overlap operator. The results support the picture of chiral symmetry breaking being closely related to instantons.

hep-lat↗
arXiv1999

Random Matrix Model for Wilson Fermions on the Lattice

We describe a random matrix model suitable for the simulation of the eigenvalues of the Dirac operator on the lattice for Wilson fermions. We compare the obtained global eigenvalue spectrum for various values of the hopping parameter κwith lattice results of Kalkreuter. The agreement is surprisingly good.

hep-ph↗
arXiv1998

IR-Renormalon Contributions to the Structure Functions $g_3$ and $g_5$

We calculate the leading $1/N_f$ perturbative contributions to the polarized nonsinglet structure functions $g_3$ and $g_5$ to all orders in $α_s$. The contributions from the first renormalon pole are determined. It is a measure for the ambiguity of the perturbative calculation and is assumed to dominate the power corrections. The corrections $Δg_3$ and $Δg_5$ are given as functions of the Bjorken variable $x$ and turn out to be negligable. The anomalous dimensions of the leading twist operators are obtained in the next-to-leading order.

hep-ph↗