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European Physical Journal C 03/2007; · 3.63 Impact Factor
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ABSTRACT: We present a comparison of a number of iterative solvers of linear systems of equations for obtaining the fermion propagator in lattice QCD. In particular, we consider chirally invariant overlap and chirally improved Wilson (maximally) twisted mass fermions. The comparison of both formulations of lattice QCD is performed at four fixed values of the pion mass between 230MeV and 720MeV. For overlap fermions we address adaptive precision and low mode preconditioning while for twisted mass fermions we discuss even/odd preconditioning. Taking the best available algorithms in each case we find that calculations with the overlap operator are by a factor of 30-120 more expensive than with the twisted mass operator.
10/2006;
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ABSTRACT: We present a detailed design of a (P)HMC simulation algorithm for N_f = 2 + 1 + 1 maximally twisted Wilson quark flavours. The algorithm retains even/odd and mass-shift preconditionings combined with multiple Molecular Dynamics time scales for both the light mass degenerate, u and d, quarks and the heavy mass non-degenerate, s and c, quarks. Various non-standard aspects of the algorithm are discussed, among which those connected to the use of a polynomial approximation for the inverse (square root of the squared) Dirac matrix in the s and c quark sector.
10/2005;
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ABSTRACT: We present simulation results for lattice QCD with chiral fermions in small volumes, where the epsilon-expansion of chiral perturbation theory applies. Our data for the low lying Dirac eigenvalues, as well as mesonic correlation functions, are in agreement with analytical predictions. This allows us to extract values for the leading Low Energy Constants F_{pi} and Sigma. Comment: 4 pages, talk presented by W.B. at Baryons04 (Paris, October 25 - 29, 2004); one Ref. added
01/2005;
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W. Bietenholz,
S. Capitani, T. Chiarappa,
N. Christian,
M. Hasenbusch,
K. Jansen,
K. -I. Nagai,
M. Papinutto,
L. Scorzato,
S. Shcheredin,
A. Shindler,
C. Urbach,
U. Wenger,
I. Wetzorke
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ABSTRACT: We compare the behavior of overlap fermions, which are chirally invariant, and of Wilson twisted mass fermions at full twist in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of O(250 MeV) can be reached in practical applications. Our comparison is done at a fixed value of the lattice spacing a=0.123 fm. A number of quantities are measured such as hadron masses, pseudoscalar decay constants and quark masses obtained from Ward identities. We also determine the axial vector renormalization constants in the case of overlap fermions. Comment: 22 pages, 10 figures
10/2004;
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W. Bietenholz,
S. Capitani, T. Chiarappa,
M. Hasenbusch,
K. Jansen,
K. -I. Nagai,
M. Papinutto,
L. Scorzato,
S. Shcheredin,
A. Shindler,
C. Urbach,
U. Wenger,
I. Wetzorke
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ABSTRACT: We compare overlap fermions, which are chirally invariant, and Wilson twisted mass fermions in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of O(250 MeV) can be reached in practical simulations. Our comparison is done at a fixed lattice spacing a=0.123 fm. Several quantities are measured, such as hadron masses and pseudoscalar decay constants.
10/2004;
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ABSTRACT: We present a systematic comparison of various iterative methods to obtain the fermion propagator with both overlap and twisted mass fermions at fixed pion mass in the quenched approximation. Taking the best available algorithm in each case we find that calculations with the overlap operator are by a factor of 20-40 more expensive than with the twisted mass operator at the parameter values considered here. For the overlap operator we also compare the efficiency of various methods for calculating the topological index. Comment: Poster presented at Lattice2004(machines), Fermilab, June 21-26, 2004, 3 pages, 4 figures
09/2004;
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ABSTRACT: We present simulation results employing overlap fermions for the axial correlation functions in the epsilon-regime of chiral perturbation theory. In this regime, finite size effects and topology play a dominant role. Their description by quenched chiral perturbation theory is compared to our numerical results in quenched QCD. We show that lattices with a linear extent L > 1.1 fm are necessary to interpret the numerical data obtained in distinct topological sectors in terms of the epsilon-expansion. Such lattices are, however, still substantially smaller than the ones needed in standard chiral perturbation theory. However, we also observe severe difficulties at very low values of the quark mass, in particular in the topologically trivial sector. Comment: 15 pages, 6 figures, final version published in JHEP
11/2003;
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W. Bietenholz,
S. Capitani, T. Chiarappa,
M. Hasenbusch,
K. Jansen,
M. Muller-Preussker,
K. -I. Nagai,
M. Papinutto,
S. Shcheredin,
A. Shindler,
C. Urbach,
I. Wetzorke
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ABSTRACT: Quantum Chromo Dynamics is considered in a setup where the light mesons are squeezed into unphysically small boxes. We show how such a situation can be used to determine the couplings of the low energy chiral Lagrangian from lattice simulations, applying chirally invariant formulations of lattice fermions.
10/2003;
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ABSTRACT: The application of Random Matrix Theory to the Dirac operator of QCD yields predictions for the probability distributions of the lowest eigenvalues. We measured Dirac operator spectra using massless overlap fermions in quenched QCD at topological charge \nu = 0, +- 1 and +- 2, and found agreement with those predictions - at least for the first non-zero eigenvalue - if the volume exceeds about (1.2 fm)^4. Comment: 3 pages, talk presented at Lattice2003(chiral)
09/2003;
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ABSTRACT: We present a numerical pilot study of the meson correlation functions in the ϵ-regime of chiral perturbation theory (χPT). Based on simulations with overlap fermions we measured the axial and pseudo-scalar correlation functions, and we discuss the implications for the leading low energy constants in the chiral Lagrangian.
Nuclear Physics B - Proceedings Supplements.
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ABSTRACT: We discuss the potential of Ginsparg-Wilson fermion simulations in the ϵ-regime of chiral perturbation theory, regarding the determination of the leading low energy constants of the effective chiral Lagrangian. It turns out to be very hard to measure observables in the topologically trivial sector. There a huge statistics would be required, due to the frequent occurrence of very small eigenvalues. Moreover, contact with chiral perturbation theory is only established if the physical volume of the system is sufficiently large.
Nuclear Physics B - Proceedings Supplements.
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Red Report (2006),.
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Red Report (2006),.
