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A. Hasenfratz

Publications and source records attributed to A. Hasenfratz.

28 records · Page 2Linked to original sources
arXiv1999

Investigating and Optimizing the Chiral Properties of Lattice Fermion Actions

We study exceptional modes of both the Wilson and the clover action in order to understand why quenched clover spectroscopy suffers so severely from exceptional configurations. We show that, in contrast to the case of the Wilson action, a large clover coefficient can make the exceptional modes extremely localized and thus very sensitive to short distance fluctuations. We describe a way to optimize the chiral behavior of Wilson-type lattice fermion actions by studying their low energy real eigenmodes. We find a candidate action, the clover action with fat links with a tuned clover term. We present a calculation of spectroscopy and matrix elements at Wilson gauge coupling beta=5.7. When compared to simulations with the standard (nonperturbatively improved) clover action at small lattice spacing, the action shows good scaling behavior, with an apparent great reduction in the number of exceptional configurations.

hep-lat↗
arXiv1998

Instanton Content of the SU(3) Vacuum

We study the topological content of the SU(3) vacuum using a method based on RG mapping developed for SU(2) gauge theory earlier. RG mapping, in which a series of APE-smearing steps is done while tracking the observables, reduces the short range fluctuations in the gauge fields while preserving the long structure. This allows us to study the instanton size distribution and topological susceptibility for SU(3) gauge theory. We arrive at a value for the topological susceptibility $χ^{1/4}$, of 203(5) MeV. The size distribution peaks at $ρ= 0.3fm$, and is in good agreement with the prediction of the instanton liquid models.

hep-lat↗
arXiv1995

Fixed point actions for SU(3) gauge theory

We summarize our recent work on the construction and properties of fixed point (FP) actions for lattice $SU(3)$ pure gauge theory. These actions have scale invariant instanton solutions and their spectrum is exact through 1--loop, i.e. in their physical predictions there are no $a^n$ nor $g^2 a^n$ cut--off effects for any $n$. We present a few-parameter approximation to a classical FP action which is valid for short correlation lengths. We perform a scaling test of the action by computing the quantity $G = L \sqrt{σ(L)}$, where the string tension $σ(L)$ is measured from the torelon mass $μ= L σ(L)$, on lattices of fixed physical volume and varying lattice spacing $a$. While the Wilson action shows scaling violations of about ten per cent, the approximate fixed point action scales within the statistical errors for $ 1/2 \ge aT_c$.

hep-lat↗
arXiv1995

The classically perfect fixed point action for SU(3) gauge theory

In this paper (the first of a series) we describe the construction of fixed point actions for lattice $SU(3)$ pure gauge theory. Fixed point actions have scale invariant instanton solutions and the spectrum of their quadratic part is exact (they are classical perfect actions). We argue that the fixed point action is even 1--loop quantum perfect, i.e. in its physical predictions there are no $g^2 a^n$ cut--off effects for any $n$. We discuss the construction of fixed point operators and present examples. The lowest order $q {\bar q}$ potential $V(\vec{r})$ obtained from the fixed point Polyakov loop correlator is free of any cut--off effects which go to zero as an inverse power of the distance $r$.

hep-lat↗
arXiv1995

Non--perturbative tests of the fixed point action for SU(3) gauge theory

In this paper (the second of a series) we extend our calculation of a classical fixed point action for lattice $SU(3)$ pure gauge theory to include gauge configurations with large fluctuations. The action is parameterized in terms of closed loops of link variables. We construct a few-parameter approximation to the classical FP action which is valid for short correlation lengths. We perform a scaling test of the action by computing the quantity $G = L \sqrt{σ(L)}$ where the string tension $σ(L)$ is measured from the torelon mass $μ= L σ(L)$. We measure $G$ on lattices of fixed physical volume and varying lattice spacing $a$ (which we define through the deconfinement temperature). While the Wilson action shows scaling violations of about ten per cent, the approximate fixed point action scales within the statistical errors for $ 1/2 \ge aT_c \ge 1/6$. Similar behaviour is found for the potential measured in a fixed physical volume.

hep-lat↗
arXiv1994

Towards a perfect fixed point action for SU(3) gauge theory

We present an overview of the construction and testing of actions for SU(3) gauge theory which are approximate fixed points of renormalization group equations (at $β\rightarrow \infty$). Such actions are candidates for use in numerical simulations on coarse lattices.

hep-lat↗
arXiv1994

QCD thermodynamics with Wilson quarks at large kappa

We have extended our study of the high temperature transition with two flavors of Wilson quarks on 12^3 x 6 lattices to kappa=0.19. We have also performed spectrum calculations on 12^3 x 24 lattices at kappa=0.19 to find the physical lattice spacing and quark mass. At this value of kappa the transition is remarkable in that the plaquette and psi-bar-psi show a large discontinuity while the Polyakov loop changes very little. This and several other features of the transition are more suggestive of a bulk transition than a transition to a quark-gluon plasma. However, if the temperature is estimated using the rho mass as a standard, the result is about 150 MeV, in agreement with the value found for the thermal transition with Kogut-Susskind quarks. uuencoded compressed Postscript file

hep-lat↗
arXiv1993

The nature of the thermal phase transition with Wilson quarks

We describe a series of simulations of high temperature QCD with two flavors of Wilson quarks aimed at clarifying the nature of the high temperature phase found in current simulations. Most of our work is with four time slices, although we include some runs with six and eight time slices for comparison. In addition to the usual thermodynamic observables we study the quark mass defined by the divergence of the axial current and the quark propagator in the Landau gauge. We find that the sharpness of the $N_t=4$ thermal transition has a maximum around $κ=0.19$ and $6/g^2=4.8$.

hep-lat↗
arXiv1992

The Phase Diagram of a U(1) Higgs-Yukawa Model at Finite $λ$

In this paper we investigate how the phase diagram of a U(1) symmetric Higgs-Yukawa system depends on the scalar self coupling $λ$. The phase diagram of similar models with continuous symmetry were extensively studied in the infinite scalar self coupling $λ=\infty$ limit. Recent analytical and numerical calculations at zero self coupling showed qualitatively different phase diagram, raising the question of the $λ$ dependence of the phase diagram. Here we use analytical (large $N_f$, perturbative and mean field) approximations as well as numerical simulations to investigate the system.

hep-lat↗