A lattice determination of light quark masses

A fully non-perturbative lattice determination of the up/down and strange quark masses is given for quenched QCD using both, O(a) improved Wilson fermions and ordinary Wilson fermions. For the strange quark mass with O(a) improved fermions we obtain m_s"M"S (#mu# = 2 GeV) = 105(4) MeV, using the interquark force scale r_0. Due to quenching problems fits are only possible for quark masses larger than the strange quark mass. If we extrapolate our fits to the up/down quark mass we find for the average mass m_l"M"S (#mu# = 2 GeV) = 4.4(2) MeV. (orig.)23 refs.Available from TIB Hannover: RA 2999(99-097) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Perturbative renormalisation of the second moment of generalised parton distributions

We calculate the non-forward quark matrix elements of operators with two covariant derivatives needed for the renormalisation of the second moment of generalised parton distributions in one-loop lattice perturbation theory using Wilson fermions. For some representations of the hypercubic group commonly used in simulations we determine the sets of all possible mixing operators. For those representations the one-loop mixing matrices of renormalisation factors are found. Due to non-vanishing contributions of operators with external ordinary derivatives the number of contributing operators increases compared to forward matrix elements. (orig.)SIGLEAvailable from TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The polarized structure function g_2: a lattice study revisited

A recent lattice calculation of the spin-dependent structure function g_2 is revisited. It has been recognized that the twist-three operator, which gives rise to d_2, mixes non-perturbatively with operators of lower dimensions under renormalization. This changes the results substantially. (orig.)8 refs.Available from TIB Hannover: RA 2999(99-129) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Renormalization of four-fermion operators for higher twist calculations

The evaluation of the higher twist contributions to deep inelastic scattering amplitudes involves a non trivial choice of operator bases for the higher orders of the OPE expansion of the two hadronic currents. In this talk we discuss the perturbative renormalization of the four-fermion operators that appear in the above bases. (orig.)7 refs.Available from TIB Hannover: RA 2999(98-132) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A lattice determination of the second moment of the polarised valence quark distribution

We perform a Monte Carlo calculation of the second moment of the polarised valence quark distributions in the nucleon, using quenched Wilson fermions. The special feature of this moment is that it is directly accessible experimentally. At a scale of #mu##approx#2 GeV we find #DELTA#"("1")u=0.198(8), #DELTA#"("1")d=-0.0477(33). We compare these numbers with recent experimental results of the SMC colla boration. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Nucleon electromagnetic form factors on the lattice and in chiral effective field theory

We compute the electromagnetic form factors of the nucleon in quenched lattice QCD, using non-perturbatively improved Wilson fermions, and compare the results with phenomenology and chiral effective field theory. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(03-032) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

New results for non-perturbative O(a) improvement in light hadrons

We have results from light hadron simulations in quenched QCD at #beta# 6.0 and 6.2 using non-perturbatively improved Sheikholeslami-Wohlert fermions in an effort to remove all O(a) effects. From looking at hadron masses and splittings and the RG-invariant quark masses (where we have one point at #beta# = 5.7) we suggest this is plausible even with the limited data set. The interpretation of the decay constants appears to be less clear. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(97-198) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Higher-twist contribution to pion structure function: 4-Fermi operators

We present quenched lattice QCD results for the contribution of higher-twist operators to the lowest non-trivial moment of the pion structure function. To be specific, we consider the combination F_2"#pi#""+ + F_2"#pi#""- - 2F_2"#pi#""0 which has I = 2 and receives contributions from 4-Fermi operators only. We introduce the basis of lattice operators. The renormalization of the operators is done perturbatively in the MS scheme using the 't Hooft-Veltman prescription for #gamma#_5, taking particular care of mixing effects. The contribution is found to be of O(f_#pi#"2/Q"2), relative to the leading contribution to the moment of F_2"#pi#""+. (orig.)14 refs.Available from TIB Hannover: RA 2999(99-098) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Scaling of non-perturbative O(a) improved Wilson fermions: Hadron spectrum, quark masses and decay constants

We compute the hadron mass spectrum, the quark masses and the meson decay constants in quenched lattice QCD with non-perturbatively O(a) improved Wilson fermions. The calculations are done for two values of the coupling constant, #beta# = 6.0 and 6.2, and the results are compared with the predictions of ordinary Wilson fermions. We find that the improved action reduces lattice artifacts as expected. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Accelerating the Hybrid Monte Carlo algorithm

An algorithm for separating the high- and low-frequency molecular dynamics modes in Hybrid Monte Carlo (HMC) simulations of gauge theories with dynamical fermions is presented. The separation is based on splitting the pseudo-fermion action into two parts, as was initially proposed by Hasenbusch. We propose to introduce different evolution time-scales for each part. We test our proposal in realistic simulations of two-flavor O(a) improved Wilson fermions. A speed-up of more than a factor of three compared to the standard HMC algorithm is observed in a typical run. (orig.)SIGLEAvailable from TIB Hannover: RA 2999(03-040) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman