20,957 research outputs found

    Structure Functions on the lattice

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    We report on a lattice computation of the second moment of the pion matrix element of the twist-2 non-singlet operator corresponding to the average momentum of parton densities. We apply a fully non-perturbatively evaluated running renormalization constant as well as a careful extrapolation of our results to the continuum limit. Thus the only limitation of our final result is the quenched approximation.Comment: 6 pages, talk presented at the XXXth International Conference on High Energy Physics (ICHEP 2000), July 27-August 2, 2000, Osaka, Japa

    The scope for fiscal policy : with examples from Thailand

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    Thailand;fiscal policy

    Speeding up the HMC: QCD with Clover-Improved Wilson Fermions

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    We apply a recent proposal to speed up the Hybrid-Monte-Carlo simulation of systems with dynamical fermions to two flavor QCD with clover-improvement. For our smallest quark masses we see a speed-up of more than a factor of two compared with the standard algorithm.Comment: 3 pages, lattice2002, algorithms, DESY Report-no correcte

    The PHMC algorithm for simulations of dynamical fermions: II - Performance analysis

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    We compare the performance of the PHMC algorithm with the one of the HMC algorithm in practical simulations of lattice QCD. We show that the PHMC algorithm can lead to an acceleration of numerical simulations. It is demonstrated that the PHMC algorithm generates configurations carrying small isolated eigenvalues of the lattice Dirac operator and hence leads to a sampling of configuration space that is different from that of the HMC algorithm.Comment: Latex2e file, 6 figures, 31 page

    The phase structure of a chirally invariant lattice Higgs-Yukawa model - numerical simulations

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    The phase diagram of a chirally invariant lattice Higgs-Yukawa model is explored by means of numerical simulations. The results revealing a rich phase structure are compared to analytical large Nf calculations which we performed earlier. The analytical and numerical results are in excellent agreement at large values of Nf. In the opposite case the large Nf computation still gives a good qualitative description of the phase diagram. In particular we find numerical evidence for the predicted ferrimagnetic phase at intermediate values of the Yukawa coupling constant and for the symmetric phase at strong Yukawa couplings. Emphasis is put on the finite size effects which can hide the existence of the latter symmetric phase.Comment: 14 pages, 11 figure

    Sigma-terms and axial charges for hyperons and charmed baryons

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    We present results for the σ\sigma-terms and axial charges for various hyperons and charmed baryons using Nf=2+1+1N_f=2+1+1 twisted mass fermions. For the computation of the three-point function we use the fixed current method. For one of the Nf=2+1+1N_f=2+1+1 ensembles with pion mass of 373 MeV we compare the results of the fixed current method with those obtained with a stochastic method for computing the all-to-all propagator involved in the evaluation of the three point functions.Comment: Talk presented at 31st International Symposium on Lattice Field Theory LATTICE 2013, July 29 - August 3, 2013, Mainz, Germany, PoS(LATTICE 2013)279. 7 pages and6 figure

    Scaling test of fermion actions in the Schwinger model

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    We discuss the scaling behaviour of different fermion actions in dynamical simulations of the 2-dimensional massive Schwinger model. We have chosen Wilson, hypercube, twisted mass and overlap fermion actions. As physical observables, the pion mass and the scalar condensate are computed for the above mentioned actions at a number of coupling values and fermion masses. We also discuss possibilities to simulate overlap fermions dynamically avoiding problems with low-lying eigenvalues of the overlap kernel

    A note on the practical feasibility of domain-wall fermions

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    Domain-wall fermions preserve chiral symmetry up to terms that decrease exponentially when the lattice size in the fifth dimension is taken to infinity. The associated rates of convergence are given by the low-lying eigenvalues of a simple local operator in four dimensions. These can be computed using the Ritz functional technique and it turns out that the convergence tends to be extremely slow in the range of lattice spacings relevant to large-volume numerical simulations of lattice QCD. Two methods to improve on this situation are discussed.Comment: 14 pages, talk given by P. H. at the workshop on {\em Current theoretical problems in lattice field theory}, Ringberg, German

    How the PHMC algorithm samples configuration space

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    We show that in practical simulations of lattice QCD with two dynamical light fermion species the PHMC algorithm samples configuration space differently from the commonly used HMC algorithm.Comment: 3 pages, 2 figures, LATTICE98 (Algorithms

    Non-perturbative running of the average momentum of non-singlet parton densities

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    We determine non-perturbatively the anomalous dimensions of the second moment of non-singlet parton densities from a continuum extrapolation of results computed in quenched lattice simulations at different lattice spacings. We use a Schr\"odinger functional scheme for the definition of the renormalization constant of the relevant twist-2 operator. In the region of renormalized couplings explored, we obtain a good description of our data in terms of a three-loop expression for the anomalous dimensions. The calculation can be used for exploring values of the coupling where a perturbative expansion of the anomalous dimensions is not valid a priori. Moreover, our results provide the non-perturbative renormalization constant that connects hadron matrix elements on the lattice, renormalized at a low scale, with the experimental results, renormalized at much higher energy scales.Comment: Latex2e file, 6 figures, 25 pages, Corrected errors on linear fit in table 2 and discussion on anomalous dimension of f_
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