arXiv subjects

Tilo Wettig

Publications and source records attributed to Tilo Wettig.

87 records · Page 5Linked to original sources

arXiv2006

Nonzero chemical potential in the overlap Dirac operator and comparison to random matrix theory

In this talk we present the results published recently in Ref. [1], where we showed how to introduce a quark chemical potential in the overlap Dirac operator. The resulting operator satisfies a Ginsparg-Wilson relation and has exact zero modes. It is no longer gamma_5-Hermitian, but its nonreal eigenvalues still occur in pairs. We compute the spectral density of the operator on the lattice and show that, for small eigenvalues, the data agree with analytical predictions of non-Hermitian chiral random matrix theory for both trivial and nontrivial topology. We also explain an observed change in the number of zero modes as a function of chemical potential.

hep-lat↗

arXiv2006

Overlap Dirac operator at nonzero chemical potential and random matrix theory

We show how to introduce a quark chemical potential in the overlap Dirac operator. The resulting operator satisfies a Ginsparg-Wilson relation and has exact zero modes. It is no longer gamma_5-hermitian, but its nonreal eigenvalues still occur in pairs. We compute the spectral density of the operator on the lattice and show that, for small eigenvalues, the data agree with analytical predictions of nonhermitian chiral random matrix theory for both trivial and nontrivial topology.

hep-lat↗

arXiv2005

Dirac eigenvalue correlations in quenched QCD at finite density

We compare eigenvalue correlations of the Dirac operator with a chemical potential obtained from lattice simulations of quenched QCD with analytic predictions obtained from chiral effective theories in the zero-momentum limit. By comparing the density and two-point correlation function we show that the analytic results agree with QCD at low energies. We also examine the scale (Thouless energy) up to which the zero-momentum approximation is valid.

hep-lat↗

arXiv2005

Performance of machines for lattice QCD simulations

We review the architecture of massively parallel machines used for lattice QCD simulations and present benchmarks for the performance of popular algorithms on these platforms. We cover commercial supercomputers, PC clusters, and custom-designed machines. We also speculate on future developments.

hep-lat↗

arXiv2004

Bosonic color-flavor transformation for the special unitary group

We extend Zirnbauer's color-flavor transformation in the bosonic sector to the color group SU(N_c). Because the flavor group U(N_b, N_b) is non-compact, the algebraic method by which the original color-flavor transformation was derived leads to a useful result only for 2N_b \le N_c. Using the character expansion method, we obtain a different form of the transformation in the extended range N_b \le N_c. This result can also be used for the color group U(N_c). The integrals to which the transformation can be applied are of relevance for the recently proposed boson-induced lattice gauge theory.

hep-lat↗

arXiv1999

Random Matrix Theory and Dirac Spectrum at Nonzero Temperature and Density

We investigate the eigenvalue spectrum of the staggered Dirac matrix in SU(3) gauge theory and in full QCD as well as in quenched U(1) theory on various lattice sizes. As a measure of the fluctuation properties of the eigenvalues, we study the nearest-neighbor spacing distribution, $P(s)$. We further study lattice QCD at nonzero chemical potential, $μ$, by constructing the spacing distribution of adjacent eigenvalues in the complex plane. We find that in all regions of their phase diagrams, compact lattice gauge theories have bulk spectral correlations given by random matrix theory, which is an indication for quantum chaos.

hep-lat↗

arXiv1999

U(1) staggered Dirac operator and random matrix

We investigate the spectrum of the staggered Dirac operator in 4d quenched U(1) lattice gauge theory and its relationship to random matrix theory. In the confined as well as in the Coulomb phase the nearest-neighbor spacing distribution of the unfolded eigenvalues is well described by the chiral unitary ensemble. The same is true for the distribution of the smallest eigenvalue and the microscopic spectral density in the confined phase. The physical origin of the chiral condensate in this phase deserves further study.

hep-lat↗

arXiv1999

Predictions of chiral random matrix theory and lattice QCD results

Chiral random matrix theory makes very detailed predictions for the spectral correlations of the QCD Dirac operator, both in the bulk of the spectrum and near zero virtuality. These predictions have been successfully tested in lattice QCD simulations by several groups. Moreover, the domain of validity of random matrix theory has been predicted theoretically and identified in lattice data. In this talk, the current numerical evidence is reviewed.

hep-lat↗

arXiv1999

Spectral correlations of the massive QCD Dirac operator at finite temperature

We use the graded eigenvalue method, a variant of the supersymmetry technique, to compute the universal spectral correlations of the QCD Dirac operator in the presence of massive dynamical quarks. The calculation is done for the chiral Gaussian unitary ensemble of random matrix theory with an arbitrary Hermitian matrix added to the Dirac matrix. This case is of interest for schematic models of QCD at finite temperature.

hep-th↗

arXiv1998

Statistical properties at the spectrum edge of the QCD Dirac operator

The statistical properties of the spectrum of the staggered Dirac operator in an SU(2) lattice gauge theory are analyzed both in the bulk of the spectrum and at the spectrum edge. Two commonly used statistics, the number variance and the spectral rigidity, are investigated. While the spectral fluctuations at the edge are suppressed to the same extent as in the bulk, the spectra are more rigid at the edge. To study this effect, we introduce a microscopic unfolding procedure to separate the variation of the microscopic spectral density from the fluctuations. For the unfolded data, the number variance shows oscillations of the same kind as before unfolding, and the average spectral rigidity becomes larger than the one in the bulk. In addition, the short-range statistics at the origin is studied. The lattice data are compared to predictions of chiral random-matrix theory, and agreement with the chiral Gaussian Symplectic Ensemble is found.

hep-lat↗

arXiv1997

Universal spectral correlations of the Dirac operator at finite temperature

Using the graded eigenvalue method and a recently computed extension of the Itzykson-Zuber integral to complex matrices, we compute the $k$-point spectral correlation functions of the Dirac operator in a chiral random matrix model with a deterministic diagonal matrix added. We obtain results both on the scale of the mean level spacing and on the microscopic scale. We find that the microscopic spectral correlations have the same functional form as at zero temperature, provided that the microscopic variables are rescaled by the temperature-dependent chiral condensate.

hep-th↗

arXiv1996

The chiral phase transition in a random matrix model with molecular correlations

The chiral phase transition of QCD is analyzed in a model combining random matrix elements of the Dirac operator with specially chosen non-random ones. The special form of the latter is motivated by the assumption that the fermionic quasi-zero modes associated with instanton and anti-instanton configurations determine the chiral properties of QCD. Our results show that the degree of correlation between these modes plays the decisive role. To reduce the value of the chiral condensate by more than a factor of 2 about 95 percent of the instantons and anti-instantons must form so-called molecules. This conclusion agrees with numerical results of the Stony Brook group.

hep-ph↗

arXiv1995

Interaction-induced delocalization of quasiparticle pairs in the Anderson insulator

It has recently been established that a short-range interaction can strongly delocalize a pair of particles moving in a disordered potential. We investigate whether an analogous effect exists also for pairs of quasiparticles in the Anderson insulator by employing an approximate numerical evaluation of the two-particle Green function of the many-body system. At the Fermi energy, the quasiparticle pair is localized on the scale of the single-particle localization length. The delocalization effect is recovered when the excitation energy of the pair is comparable to the bandwidth.

cond-mat↗

arXiv1995

Interaction-induced delocalization of two particles in a random potential: Scaling properties

The localization length $ξ_2$ for coherent propagation of two interacting particles in a random potential is studied using a novel and efficient numerical method. We find that the enhancement of $ξ_2$ over the one-particle localization length $ξ_1$ satisfies the scaling relation $ξ_2/ξ_1=f(u/Δ_ξ)$, where $u$ is the interaction strength and $Δ_ξ$ the level spacing of a wire of length $ξ_1$. The scaling function $f$ is linear over the investigated parameter range. This implies that $ξ_2$ increases faster with $u$ than previously predicted. We also study a novel mapping of the problem to a banded-random-matrix model.

cond-mat↗

arXiv1994

Criticality in Simple Models of Evolution

We consider two, apparently similar, models of biological evolution which have been claimed to exhibit self-organized critical behaviour. A careful reanalysis of these models, including several new analytic results for one of them, suggests that they are qualitatively different. We demonstrate the limitations of the mean field description of these systems. We argue that a more precise definition of self-organized criticality is desirable and establish several criteria in this connection.

cond-mat↗