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C. B. Lang

Publications and source records attributed to C. B. Lang.

At least 91 records · Page 5Linked to original sources
arXiv2007

The eigenvalue spectrum for dynamical Chirally Improved fermions

We study the eigenvalues of Dirac operators in QCD with two mass degenerate dynamical fermions. The gauge configurations have been obtained with HMC and the so-called Chirally Improved fermionic action. We compare eigenvalues obtained for the overlap Dirac operator on these configurations with those for the Chirally Improved (CI) operator (studied earlier). Results of Random Matrix Theory allow us to determine the chiral condensate.

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arXiv2007

Meson spectroscopy with derivative quark sources

We present results for masses of light mesons obtained with the variational method using an enhanced basis of interpolating field operators with different quark smearings. The interpolators are constructed from Jacobi-smeared quarks of a Gaussian type as well as from derivative quark sources obtained by a covariant derivative acting on the Gaussian sources. For our analysis we use quenched gauge configurations with Chirally Improved quarks and the Luescher-Weisz gauge action on a 16^3 x 32 lattice with a = 0.148 fm. We discuss the influence of derivative sources on the overlap with excited states.

hep-lat↗
arXiv2007

The condensate for two dynamical chirally improved quarks in QCD

We compare the eigenvalue spectra of the Dirac operator from a simulation with two mass degenerate dynamical chirally improved fermions with Random Matrix Theory. Comparisons with distribution of k-th eigenvalues (k=1,2) in fixed topological sectors (nu=0,1) are carried out using the Kolmogorov-Smirnov test. The eigenvalue distributions are well described by the RMT predictions. The match allows us to read off the quark condensate in the chiral limit directly. Correcting for finite size and renormalization we obtain a mean value of -(276 (11)(16) MeV)**3 in the MS-bar scheme.

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↗
arXiv2006

QCD with two dynamical flavors of chirally improved quarks

Considering Ginsparg-Wilson type fermions dynamically in lattice QCD simulations is a challenging task. The hope is to be able to approach smaller pion masses and to eventually reach physical situations. The price to pay is substantially higher computational costs. Here we discuss first results of a dynamical implementation of the so-called Chirally Improved Fermions, a Dirac operator that obeys the Ginsparg-Wilson condition approximately. The simulation is for two species of mass-degenerate quarks on 12^3x24 lattices with spatial size up to 1.55 fm. Implementation of the Hybrid Monte-Carlo algorithm and an analysis of the results are presented.

hep-lat↗
arXiv2006

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.

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arXiv2006

Dirac Eigenmodes in an Environment of Dynamical Fermions

We discuss some properties of zero and near-zero modes of the Dirac operator, as observed in a recent simulation of 2-flavor QCD. The quarks have been implemented with the so-called Chirally Improved Dirac operator, which obeys the Ginsparg-Wilson relation to a good approximation. We present geometrical and statistical properties of these eigenmodes and eigenvalues.

hep-lat↗
arXiv2005

Variational method for lattice spectroscopy with ghosts

We discuss the variational method used in lattice spectroscopy calculations. In particular we address the role of ghost contributions which appear in quenched or partially quenched simulations and have a non-standard euclidean time dependence. We show that the ghosts can be separated from the physical states. Our result is illustrated with numerical data for the scalar meson.

hep-lat↗
arXiv2005

Lattice calculation of low energy constants with Ginsparg-Wilson type fermions

We present a quenched lattice calculation of low energy constants using the chirally improved Dirac operator. Several lattice sizes at different lattice spacings are studied. We systematically compare various methods for computing these quantities, using pseudoscalar and axial vector correlators. We find consistent results for the different approaches, giving rise to f_π= 96(2)(4) MeV, f_K = 106(1)(8) MeV, f_K/f_π=1.11(1)(2), Sigma= -(286(4)(31) MeV)^3, the average light quark mass m = 4.1(2.4) MeV and m_s = 101(8) MeV.

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arXiv2005

Low energy constants from the chirally improved Dirac operator D_CI

The leading order low energy parameters like the pion decay constant or the quark condensate are well-known from "classical" low energy theorems and experiments. It is a challenge, however, to find these parameters based exclusively on an ab-initio QCD calculation. We discuss results of a quenched lattice calculation of low energy constants using the chirally improved Dirac operator. Several lattice sizes at different lattice spacings are studied, using pseudoscalar and axial vector correlators. We find consistent results for f_π= 96(2) MeV, f_K = 105(2) MeV, Σ= -(286(4) MeV)^3, the average light quark mass m = 4.1(2.4) MeV and m_s = 101(8) MeV.

hep-lat↗
arXiv2005

Excited meson spectroscopy with chirally improved fermions

We present excited meson masses from quenched calculations using chirally improved (CI) quarks at pion masses down to 350 MeV. The salient 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↗
arXiv2005

Baryon spectroscopy with spatially improved quark sources

We study baryons on the lattice with a special focus on excited states. For that purpose we construct several interpolators which differ in their Dirac structure. These interpolators are built from Jacobi smeared quarks with different widths in order to allow for operators with improved spatial wavefunctions. We compute all cross correlations and use the variational method to determine which combinations of operators have best overlap with ground and excited states. Our approach yields promising results for the spin-1/2 baryons: nucleon, sigma, xi and lambda. For the spin-3/2 baryons, delta and omega, we obtain results which are consistent with results of other groups.

hep-lat↗
arXiv2005

Pion form factor with chirally improved fermions

We present results for Monte Carlo calculations of the electromagnetic vector and scalar form factors of the pion in a quenched simulation. We work at a lattice spacing of 0.15 fm and use two lattice volumes up to a spatial size of 2.4 fm. The pion form factors in the space-like region are determined for pion masses down to 340 MeV.

hep-lat↗
arXiv2005

Implementing Hybrid Monte Carlo with stout-smeared chirally improved Dirac operators

We discuss our implementation of dynamical Ginsparg-Wilson type fermions using a stout-smeared chirally improved Dirac operator. Such operators have been studied extensively in quenched calculations within the Bern-Graz-Regensburg (BGR) collaboration. Here we discuss the development and testing of the Hybrid Monte Carlo algorithm with this Dirac operator. We study the chiral properties of this operator in a dynamical setup, comparing, e.g., the spectra of the operator for the dynamical and quenched cases. We then discuss quantitative features of the algorithm like autocorrelation and performance.

hep-lat↗
arXiv2005

First results from dynamical chirally improved fermions

We simulate Quantum Chromodynamics in four Euclidean dimensions with two (degenerate mass) flavors of dynamical quarks. The Dirac operator is the so-called chirally improved operator that has been studied so far in quenched calculations. We now present results of an implementation with the Hybrid Monte Carlo (HMC) algorithm including stout smearing. Our results are from an 8^3x16 lattice with tadpole improved Luescher-Weisz gauge action. We present our estimate of the lattice spacing, the pi and rho meson masses and evidence for tunneling between different topological sectors.

hep-lat↗
arXiv2005

Experiences with dynamical chirally improved fermions

We simulate Quantum Chromodynamics (QCD) in four Euclidean dimensions with two (degenerate mass) flavors of dynamical quarks. The Dirac operator we use is the so-called chirally improved Dirac operator. We discuss the algorithm used for the simulation as well as the checks and some results on lattices up to size 8**4 for fermion mass parameters down to 0.1. This is the first attempt to introduce dynamical quarks with the chirally improved Dirac operator.

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