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F. Knechtli

Publications and source records attributed to F. Knechtli.

At least 19 recordsLinked to original sources
arXiv2008

O(a) improvement of the HYP static axial and vector currents at one-loop order of perturbation theory

We calculate analytically the improvement coefficients of the static axial and vector currents in O(a) improved lattice QCD at one-loop order of perturbation theory. The static quark is described by the hypercubic action, previously introduced in the literature in order to improve the signal-to-noise ratio of static observables. Within a Schroedinger Functional setup, we derive the Feynman rules of the hypercubic link in time-momentum representation. The improvement coefficients are obtained from on-shell correlators of the static axial and vector currents. As a by-product, we localise the minimum of the static self-energy as a function of the smearing parameters of the action at one-loop order and show that the perturbative minimum is close to its non-perturbative counterpart.

hep-lat↗
arXiv2007

New Higgs mechanism from the lattice

Spontaneous symmetry breaking has been observed in lattice simulations of five-dimensional gauge theories on an orbifold. This effect is reproduced by perturbation theory if it is modified to account for a finite cut-off. We present a comparison of lattice and analytic results for bulk gauge group SU(2).

hep-ph↗
arXiv2006

Non-perturbative mass spectrum of an extra-dimensional orbifold

We analyse non-perturbatively a five-dimensional SU(2) gauge theory compactified on the S^1/Z_2 orbifold. In particular, we present simulation results for the mass spectrum of the theory, which contains a Higgs and a photon. The Higgs mass is found to be free of divergences without fine-tuning. The photon mass is non-zero, thus providing us with the first lattice evidence for a Higgs mechanism derived from an extra dimension. Data from the static potential are consistent with dimensional reduction at low energies.

hep-lat↗
arXiv2004

Universality in the Gross-Neveu model

We consider universal finite size effects in the large-N limit of the continuum Gross-Neveu model as well as in its discretized versions with Wilson and with staggered fermions. After extrapolation to zero lattice spacing the lattice results are compared to the continuum values.

hep-lat↗
arXiv2004

Cutoff-effects in the spectrum of dynamical Wilson fermions

We investigate the low-lying eigenvalues of the improved Wilson-Dirac operator in the Schroedinger functional with two dynamical quark flavors. At a lattice spacing of approximately 0.1 fm we find more very small eigenvalues than in the quenched case. These cause problems with HMC-type algorithms and in the evaluation of fermionic correlation functions. Through a simulation at a finer lattice spacing we are able to establish their nature as cutoff-effects

hep-lat↗
arXiv2004

The locality problem for two tastes of staggered fermions

We address the locality problem arising in simulations, which take the square root of the staggered fermion determinant as a Boltzmann weight to reduce the number of dynamical quark tastes from four to two. We study analytically and numerically the square root of the staggered fermion operator as a candidate to define a two taste theory from first principles. Although it has the correct weight, this operator is non-local in the continuum limit. Our work serves as a warning that fundamental properties of field theories might be violated when employing blindly the square root trick. The question, whether a local operator reproducing the square root of the staggered fermion determinant exists, is left open.

hep-lat↗
arXiv2004

Locality with staggered fermions

We address the locality problem arising in simulations, which take the square root of the staggered fermion determinant as a Boltzmann weight to reduce the number of dynamical quark tastes. A definition of such a theory necessitates an underlying local fermion operator with the same determinant and the corresponding Green's functions to establish causality and unitarity. We illustrate this point by studying analytically and numerically the square root of the staggered fermion operator. Although it has the correct weight, this operator is non-local in the continuum limit. Our work serves as a warning that fundamental properties of field theories might be violated when employing blindly the square root trick. The question, whether a local operator reproducing the square root of the staggered fermion determinant exists, is left open.

hep-lat↗
arXiv2003

Large cutoff effects of dynamical Wilson fermions

We present and discuss results for cutoff effects in the PCAC masses and the mass dependence of r_0 for full QCD and various fermion actions. Our discussion of how one computes mass dependences - here of r_0 - is also relevant for comparisons with chiral perturbation theory.

hep-lat↗
arXiv2003

Non-perturbative renormalization of the axial current with improved Wilson quarks

We present a new normalization condition for the axial current, which is derived from the PCAC relation with non-vanishing mass. Using this condition reduces the O(r_0 m) corrections to the axial current normalization constant Z_A for an easier chiral extrapolation in the cases, where simulations at zero quark-mass are not possible. The method described here also serves as a preparation for a determination of Z_A in the full two-flavor theory.

hep-lat↗
arXiv2003

Simulating the Schroedinger functional with two pseudo-fermions: algorithmic study and the running mass

We present an algorithmic study for the simulation of two massless flavors of O(a) improved Wilson quarks with Schroedinger functional boundary conditions. The algorithm used is Hybrid Monte Carlo with two pseudo-fermion fields as proposed by M. Hasenbusch. A gain in CPU cost of a factor two is reached when compared to one pseudo-fermion field due to the larger possible step-size. This study is integrated in the ALPHA project for the computation of the running of the renormalized quark mass. We include an update on these physics results.

hep-lat↗
arXiv2003

Comparison of analytic and numerical results in the XY-model

We study the two dimensional XY-model with high precision Monte Carlo techniques and investigate the continuum approach of the step-scaling function of its finite volume mass gap. The continuum extrapolated results are found consistent with analytic predictions for the finite volume energy spectrum based on the equivalence with sine-Gordon theory. To come to this conclusion it was essential to use an also predicted form of logarithmic decay of lattice artifacts for the extrapolation.

hep-lat↗
arXiv2003

Simulating the Schroedinger functional with two pseudo-fermions

We report on simulations with two flavors of O(a) improved degenerate Wilson fermions with Schroedinger functional boundary conditions. The algorithm which is used is Hybrid Monte Carlo with two pseudo-fermion fields as proposed by M. Hasenbusch. We investigate the numerical precision and sensitivity to reversibility violations of this algorithm. A gain of a factor two in CPU cost is reached compared with one pseudo-fermion field due to the larger possible step-size.

hep-lat↗
arXiv2002

Recent results on the running coupling in QCD with two massless flavours

We report on the latest results on the running coupling of two flavour QCD in the Schrödinger functional scheme. Results for the step scaling function are obtained from simulations on lattices $L/a=8$ and $L/a=16$ which confirm the first results from lattices $L/a=4,5,6$ presented one year ago by the ALPHA collaboration. We also discuss some algorithmic aspects, in particular concerning the occurrence of metastable states. A modified sampling, in order to estimate the proper weight of these states in the path integral, is proposed and tested.

hep-lat↗
arXiv2002

Running quark mass in two flavor QCD

We present first results for the step scaling function sigma_P of the renormalization factor Z_P of the pseudoscalar density. The simulations are performed within the framework of the Schroedinger functional with two flavors of O(a) improved Wilson fermions. The knowledge of sigma_P is required to compute the renormalization group invariant quark masses. We also study the performance of a variant of the HMC algorithm using two pseudofermion fields.

hep-lat↗
arXiv2002

Simulation of dynamical fermions with smeared links

Smearing the gauge links of dynamical configurations removes small scale unphysical vacuum fluctuations und thus improves the chiral properties of lattice fermions. We present a new algorithm for the simulation of dynamical fermions coupled via smeared links based on the standard pure gauge overrelaxation and heatbath updatings. Smeared links play a fundamental role in making this algorithm effective. At fixed lattice spacing the computational cost of the algorithm has an extra volume factor due to the finite volume of the lattice region which can be updated. As the continuum limit is approached the physical volume of the updated region remains constant. We simulated four flavors of staggered fermions coupled via hypercubic (HYP) smeared links. The simulation cost of the new algorithm on 10 fm^4 volumes is a factor 2-8 larger than with the standard Hybrid Monte Carlo but the improved properties of the HYP action allow to gain a factor 2 in the lattice spacing. The new algorithm could be applicable to simulations of more complicated chiral fermionic actions, like overlap or perfect actions.

hep-lat↗
arXiv2001

The Static Potential with Hypercubic Blocking

We measure the static potential from Wilson loops constructed using hypercubic blocked (HYP) links. The HYP potential agrees with the potential measured using thin links for distances r/a>=2. We calculated the lowest order perturbative expansion of the lattice Coulomb potential of HYP links. These results are used in analyzing the static potential both on quenched and dynamical lattices. The statistical accuracy of the potential with HYP links improves by about an order of magnitude, giving a reliable scale even with limited statistics both on quenched and dynamical lattices.

hep-lat↗
arXiv2001

Simulating dynamical fermions with smeared links

Smearing the gauge links of dynamical configurations removes small scale unphysical vacuum fluctuations and thus improves the chiral properties of lattice fermions. Recently we proposed the hypercubic smearing (HYP) that improves the flavor symmetry of staggered fermions by an order of magnitude with only minimal distortions at small distances. We describe a new algorithm to simulate dynamical HYP fermions based on the standard pure gauge overrelaxation and heatbath updates. The algorithm has been used to simulate four and two flavors of staggered fermions. Unlike standard dynamical simulation techniques, this algorithm does not loose efficiency at small quark masses.

hep-lat↗