One-loop renormalisation of quark bilinears for overlap fermions with improved gauge actions

We compute lattice renormalisation constants of local bilinear quark operators for overlap fermions and improved gauge actions. Among the actions we consider are the Symanzik, L\"uscher-Weisz, Iwasaki and DBW2 gauge actions. The results are given for a variety of $\rho$ parameters. We show how to apply mean field (tadpole) improvement to overlap fermions. The question, what is a good gauge action, is discussed from the perturbative point of view. Finally, we show analytically that the gauge dependent part of the self-energy and the amputated Green functions are independent of the lattice fermion representation, using either Wilson or overlap fermions.Comment: 38 pages, 5 figures, v2: Numbers in Tables 1-4,7-10 corrected, Figs. 4,5 updated, 2 misprints in (A.2), (A.4) correcte

Generalized Parton Distributions in Full Lattice QCD

We present recent results on generalized parton distributions from dynamical lattice QCD calculations. Our set of twelve different combinations of couplings and quark masses allows for a preliminary study of the pion mass dependence of the transverse nucleon structure.Comment: 8 pages, 5 figures; Talk presented by Ph.H. at Light-Cone 2004, Amsterdam, 16 - 20 Augus

Improved determination of hadron matrix elements using the variational method

The extraction of hadron form factors in lattice QCD using the standard two- and three-point correlator functions has its limitations. One of the most commonly studied sources of systematic error is excited state contamination, which occurs when correlators are contaminated with results from higher energy excitations. We apply the variational method to calculate the axial vector current gA and compare the results to the more commonly used summation and two-exponential fit methods. The results demonstrate that the variational approach offers a more efficient and robust method for the determination of nucleon matrix elements.Comment: 7 pages, 6 figures, talk presented at Lattice 2015, PoS (LATTICE2015

Nucleon distribution amplitudes from lattice QCD

We calculate low moments of the leading-twist and next-to-leading twist nucleon distribution amplitudes on the lattice using two flavors of clover fermions. The results are presented in the MSbar scheme at a scale of 2 GeV and can be immediately applied in phenomenological studies. We find that the deviation of the leading-twist nucleon distribution amplitude from its asymptotic form is less pronounced than sometimes claimed in the literature.Comment: 5 pages, 3 figures, 2 tables. RevTeX style. Normalization for \lambda_i corrected. Discussion of the results extended. To be published in PR

Structure functions from the Compton amplitude

We have initiated a program to compute the Compton amplitude from lattice QCD with the Feynman-Hellman method. This amplitude is related to the structure function via a Fredholm integral equation of the first kind. It is known that these types of equations are inherently ill--posed - they are, e.g., extremely sensitive to perturbations of the system. We discuss two methods which are candidates to handle these problems: the model free inversion based on singular value decomposition and one Bayesian type approach. We apply the Bayesian method to currently available lattice data for the Compton amplitude

Measurement of the Cross Section of W-boson pair production at LEP

The cross section of W-boson pair-production is measured with the L3 detector at LEP. In a data sample corresponding to a total luminosity of 629.4/pb, collected at centre-of-mass energies ranging from 189 to 209 GeV, 9834 four-fermion events with W bosons decaying into hadrons or leptons are selected. The total cross section is measured with a precision of 1.4 % and agrees with the Standard Model expectation. Assuming charged-lepton universality, the branching fraction for hadronic W-boson decays is measured to be: Br(W-->hadrons) = 67.50 +- 0.42 (stat.) +- 0.30(syst.) %, in agreement with the Standard Model. Differential cross sections as a function of the W- production angle are also measured for the semi-leptonic channels qqev and qqmv