Generalized parton distributions and nucleon structure at large x from lattice QCD

Recent progress in lattice calculations of generalized parton distributions is discussed, with emphasis on the large-x behavior of nucleon structure functions. (orig.)Available from TIB Hannover: RA 2999(04-212) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Actions for dynamical fermion simulations: Are we ready to go?

A critical review, playing devil's advocate, on present dynamical fermion simulations is given. (orig.)Available from TIB Hannover: RA 2999(03-192) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Speeding up the Hybrid-Monte-Carlo algorithm for dynamical fermions

We propose a modification of the Hybrid-Monte-Carlo algorithm that allows for a larger step-size of the integration scheme at constant acceptance rate. The key ingredient is that the pseudo-fermion action is split into two parts. We test our proposal at the example of the two-dimensional lattice Schwinger model with two degenerate flavours of Wilson-fermions. (orig.)Available from TIB Hannover: RA 2999(01-103) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The topological susceptibility with dynamical overlap fermions

Using a reweighting technique combined with a low-mode truncation of the fermionic determinant, we perform the first computation of the quark-mass dependence of the QCD topological susceptibility with overlap fermions. In contrast to previous lattice simulations which all used non-chiral fermions, our results are in good agreement with the simple continuum model of Duerr. This demonstrates that chirality of the lattice fermion action is crucial for obtaining the expected suppression of the susceptibility. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(01-211) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Thermal dilepton and meson spectral functions from lattice QCD

Pseudo-scalar and vector meson correlation functions were calculated at temperatures below and above the deconfinement transition using O(a) improved Wilson fermions in quenched lattice QCD. The spectral functions were reconstructed from the correlator given only at discrete points in Euclidean time by means of the Maximum Entropy Method without a priori assumptions on the spectral shape. The deviation of the spectral functions from the one of a freely propagating quark anti-quark pair is examined above the critical temperature. At 1.5 and 3 T_c the vector spectral function yields an enhancement of the dilepton rate over the Born rate in the energy interval 4 < #omega#/T < 8 and a sharp drop at energies below 2-3 T. (orig.)Available from TIB Hannover: RA 2999(02-19) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Unquenching the topological susceptibility with an overlap action

We estimate the quark-mass dependence of the topological susceptibility with dynamical overlap and clover fermions. Unquenching effects on the susceptibility turn out to be well approximated by a reweighting of a quenched ensemble with a low-eigenmode truncation of the fermionic determinant. We find that it is most likely due to the explicit chiral symmetry breaking of the fermion action that present day dynamical simulations do not show the expected suppression of the topological susceptibility. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(01-209) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Full QCD algorithms towards the chiral limit

I discuss the behaviour of algorithms for dynamical fermions as the sea-quark mass decreases. I focus on the Hybrid-Monte-Carlo (HMC) algorithm applied to two degenerate flavours of Wilson fermions. First, I briefly review the performance obtained in large scale HMC simulation. Then I discuss a modified pseudo-fermion action for the HMC simulation that has been introduced three years ago. I summarize recent results obtained with this pseudo-fermion action by the QCDSF and the ALPHA collaborations. I comment on alternatives to the HMC, like the Multiboson algorithm and variants of it. (orig.)Available from TIB Hannover: RA 2999(03-124) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Lattice perturbation theory

The consideration of quantum fields defined on a spacetime lattice provides computational techniques which are invaluable for studying gauge theories nonperturbatively from first principles. Perturbation theory is an essential aspect of computations on the lattice, especially for investigating the behavior of lattice theories near the continuum limit. Particularly important is its role in connecting the outcome of Monte Carlo simulations to continuum physical results. For these matchings the calculation of the renormalization factors of lattice matrix elements is required. In this review we explain the main methods and techniques of lattice perturbation theory, focusing on the cases of Wilson and Ginsparg-Wilson fermions. We illustrate, among other topics, the peculiarities of perturbative techniques on the lattice, the use of computer codes for the analytic calculations and the computation of lattice integrals. Discussed are also methods for the computation of 1-loop integrals with very high precision. The review presents in a pedagogical fashion also some of the recent developments in this kind of calculations. The coordinate method of Luescher and Weisz is explained in detail. Also discussed are the novelties that Ginsparg-Wilson fermions have brought from the point of view of perturbation theory. Particular emphasis is given throughout the paper to the role of chiral symmetry on the lattice and to the mixing of lattice operators under renormalization. The construction of chiral gauge theories regularized on the lattice, made possible by the recent advances in the understanding of chiral symmetry, is also discussed. Finally, a few detailed examples of lattice perturbative calculations are presented. (orig.)Available from TIB Hannover: RA 2999(02-185) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Status of lattice structure function calculations

The moments of structure functions can be calculated from first principles, without making any model assumptions, using the non-perturbative techniques of lattice QCD. Numerous results have been obtained from lattice computations in the last years. They include the calculation of the lowest moments of the unpolarized structure functions, of the spin-dependent g_1 and g_2 structure functions and of the h_1 transversity structure function. Some higher-twist matrix elements have been studied as well. The novelties of the last couple of years are the first computations of structure functions done in full QCD (unquenched), and new proposals for the extrapolations to the chiral limit using chiral perturbation theory. A lot of work however still needs to be done in order to control other systematic uncertainties like the continuum limit or the non-perturbative renormalization. (orig.)Available from TIB Hannover: RA 2999(02-067) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Overlap and domain wall fermions: what is the price of chirality?

In this contribution the costs of simulations employing domain wall and overlap fermions are estimated. In the discussion we stay within the quenched approximation. (orig.)Available from TIB Hannover: RA 2999(01-164) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman