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M. Müller-Preussker

Publications and source records attributed to M. Müller-Preussker.

At least 37 records · Page 2Linked to original sources
arXiv2007

Calorons and dyons at the thermal phase transition analyzed by overlap fermions

In a pilot study, we use the topological charge density defined by the eigenmodes of the overlap Dirac operator (with ultraviolet filtering by mode-truncation) to search for lumps of topological charge in SU(2) pure gauge theory. Augmenting this search with periodic and antiperiodic temporal boundary conditions for the overlap fermions, we demonstrate that the lumps can be classified either as calorons or as separate caloron constituents (dyons). Inside the topological charge clusters the (smeared) Polyakov loop is found to show the typical profile characteristic for calorons and dyons. This investigation, motivated by recent caloron/dyon model studies, is performed at the deconfinement phase transition for SU(2) gluodynamics on 20^3 x 6 lattices described by the tadpole improved Lüscher-Weisz action. The transition point has been carefully located. As a necessary condition for the caloron/dyon detection capability, we check that the LW action, in contrast to the Wilson action, generates lattice ensembles, for which the overlap Dirac eigenvalue spectrum smoothly behaves under smearing and under the change of the boundary conditions.

hep-lat↗
arXiv2006

Topology and confinement at T \neq 0 : calorons with non-trivial holonomy

In this talk, relying on experience with various lattice filter techniques, we argue that the semiclassical structure of finite temperature gauge fields for T < T_c is dominated by calorons with non-trivial holonomy. By simulating a dilute gas of calorons with identical holonomy, superposed in the algebraic gauge, we are able to reproduce the confining properties below T_c up to distances r = O(4 fm} >> ρ(the caloron size). We compute Polyakov loop correlators as well as space-like Wilson loops for the fundamental and adjoint representation. The model parameters, including the holonomy, can be inferred from lattice results as functions of the temperature.

hep-ph↗
arXiv2006

Landau Gauge Gluon and Ghost Propagators from Lattice QCD

We report on recent numerical computations of the Landau gauge gluon and ghost propagators as well as of the ghost-gluon vertex function in pure SU(3) Yang-Mills theory and in full QCD on the lattice. Special emphasis is paid to the low momentum region. In particular, we present new data for the gluon propagator at momenta below 300 MeV. We also discuss different systematic effects as there are finite-size, lattice discretization and Gribov copy but also unquenching effects. A MOM-scheme running coupling α_s(q^2) based on the ghost-gluon vertex is calculated and found to decrease for momenta below 550 MeV, even though the renormalization constant of the vertex deviates only weakly from being constant.

hep-lat↗
arXiv2006

Twisted mass QCD thermodynamics: first results on apeNEXT

The motivations for simulating QCD thermodynamics with Wilson fermions and a twisted mass term are introduced. The twisted mass approach provides a natural infrared cutoff and O(a) improvement at maximal twist, and can be extended to finite temperature. Our strategy for exploring the QCD phase diagram at finite temperature in this setup, while taking advantage of the results at T=0, is explained. The first results for the order parameters and susceptibilities on a 16^3 x 8 lattice are presented. All dynamical simulations are carried out on the apeNEXT facility in Rome.

hep-lat↗
arXiv2006

Lattice study of the infrared behavior of QCD Green's functions in Landau gauge

We summarize the current status of our numerical results for the gluon and ghost propagators and for the Kugo-Ojima confinement parameter in quenched SU(3) lattice Landau gauge theory. The data for the propagators are compared to our results obtained in the case of full QCD, simulated using two flavors of dynamical clover-improved Wilson fermions. We demonstrate that the infrared behavior of the ghost propagator is consistent with the Kugo-Ojima confinement criterion. Explicit violation of reflection positivity by the gluon propagator is shown. Additionally, we present results of a running coupling constant both at low and at large momenta.

hep-lat↗
arXiv2006

Once more on the interrelation between Abelian monopoles and P-vortices in SU(2) LGT

We study the properties of configurations from which P-vortices on one hand or Abelian monopoles on the other hand have been removed. We confirm the loss of confinement in both cases and investigate in what respect the modified ensembles differ from the confining ones from the point of view of the complementary confinement scenario.

hep-lat↗
arXiv2006

An SU(2) KvBLL caloron gas model and confinement

A semi-classical model is developed to describe pure SU(2) Yang-Mills gluodynamics at finite temperature as a dilute, non-interacting gas of Kraan-van Baal-Lee-Lu calorons including the case of non-trivial holonomy. Temperature dependent parameters of the model (asymptotic caloron holonomy, caloron density and caloron size distribution) are discussed from the point of view of lattice observations and of in-medium modifications of the one-loop caloron amplitude. Space-like string tensions running into plateaux at distances R \approx 0.8 - 1.3 fm are obtained and compared to lattice results in order to find more precisely the average caloron size. Then, the quark-antiquark free energy as predicted by the model is considered. In the confined phase a linear rise with the separation can be observed up to R \approx 4 fm, whereas it runs into plateaux above T_c. Screening effects in the adjoint potentials are observed together with an approximate Casimir scaling of the caloron contribution to the fundamental and adjoint forces. In Abelian projection, space-like percolation of monopoles is found in the confined phase only. Thus, taking the non-trivial holonomy into account, confinement properties of pure SU(2) Yang-Mills gluodynamics can be described by a semi-classical approach up to distances one order of magnitude larger than the caloron size.

hep-ph↗
arXiv2006

Calorons and monopoles from smeared SU(2) lattice fields at non-zero temperature

In equilibrium, at finite temperature below and above the deconfining phase transition, we have generated lattice SU(2) gauge fields and have exposed them to smearing in order to investigate the emerging clusters of topological charge. Analysing in addition the monopole clusters according to the maximally Abelian gauge, we have been able to characterize part of the topological clusters to correspond either to non-static calorons or static dyons in the context of Kraan-van Baal caloron solutions with non-trivial holonomy. We show that the relative abundance of these calorons and dyons is changing with temperature and offer an interpretation as dissociation of calorons into dyons with increasing temperature. The profile of the Polyakov loop inside the topological clusters and the (model-dependent) accumulated topological cluster charges support this interpretation. Above the deconfining phase transition light dyons (according to Kraan-van Baal caloron solutions with almost trivial holonomy) become the most abundant topological objects. They are presumably responsible for the magnetic confinement in the deconfined phase.

hep-lat↗
arXiv2006

Gauge-variant propagators and the running coupling from lattice QCD

On the occasion of the 70th birthday of Prof. Adriano Di Giacomo we report on recent numerical computations of the Landau gauge gluon and ghost propagators as well as of a non-symmetric MOM-scheme ghost-gluon vertex in quenched and full lattice QCD. Special emphasis is paid to the Gribov copy problem and to the unquenching effect. The corresponding running coupling α_s(q^2) is found and shown to decrease for q^2 \le 0.3 GeV^2 in the infrared limit. No indication for a non-trivial infrared fixed point is seen in agreement with findings from truncated systems of Dyson-Schwinger equations treated on a four-dimensional torus.

hep-lat↗
arXiv2005

Probing the Aoki phase with N_f=2 Wilson fermions at finite temperature

In this letter we report on a numerical investigation of the Aoki phase in the case of finite temperature which continues our former study at zero temperature. We have performed simulations with Wilson fermions at $β=4.6$ using lattices with temporal extension $N_τ=4$. In contrast to the zero temperature case, the existence of an Aoki phase can be confirmed for a small range in $κ$ at $β=4.6$, however, shifted slightly to lower $κ$. Despite fine-tuning $κ$ we could not separate the thermal transition line from the Aoki phase.

hep-lat↗
arXiv2005

Landau gauge ghost and gluon propagators and the Faddeev-Popov operator spectrum

In this talk we report on a recent lattice investigation of the Landau gauge gluon and ghost propagators in pure SU(3) lattice gauge theory with a special emphasis on the Gribov copy problem. In the (infrared) region of momenta $q^2 \le 0.3 \mathrm{GeV}^2$ we find the corresponding MOM scheme running coupling $α_s(q^2)$ to rise in $q$. We also report on a first SU(3) computation of the ghost-gluon vertex function showing that it deviates only weakly from being constant. In addition we study the spectrum of low-lying eigenvalues and eigenfunctions of the Faddeev-Popov operator as well as the spectral representation of the ghost propagator.

hep-lat↗
arXiv2005

Calorons in SU(3) lattice gauge theory

We examine the semiclassical content of SU(3) Yang Mills theory on the lattice at finite temperature. Employing the cooling method, a set of classical fields is generated from a Monte Carlo ensemble. Various operators are used to inspect this set with respect to topological properties. We find pseudoparticle fields, so-called caloron solutions, possessing the remarkable features of (superpositions of) Kraan-van Baal solutions, i.e. extensions of Harrington-Shepard calorons to generic values of the holonomy.

hep-lat↗
arXiv2005

The monopole content of topological clusters : have KvB calorons been found?

Using smearing of equilibrium lattice fields generated at finite temperature in the confined phase of SU(2) lattice gauge theory, we have investigated the emerging topological objects (clusters of topological charge). Analysing their monopole content according to the Polyakov gauge and the maximally Abelian gauge, we characterize part of them to correspond to nonstatic calorons or static dyons in the context of Kraan-van Baal caloron solutions with non-trivial holonomy. The behaviour of the Polyakov loop inside these clusters and the (model-dependent) topological charges of these objects support this interpretation.

hep-lat↗
arXiv2004

On the stability of Dirac sheet configurations

Using cooling for SU(2) lattice configurations, purely Abelian constant magnetic field configurations were left over after the annihilation of constituents that formed metastable Q=0 configurations. These so-called Dirac sheet configurations were found to be stable if emerging from the confined phase, close to the deconfinement phase transition, provided their Polyakov loop was sufficiently non-trivial. Here we show how this is related to the notion of marginal stability of the appropriate constant magnetic field configurations. We find a perfect agreement between the analytic prediction for the dependence of stability on the value of the Polyakov loop (the holonomy) in a finite volume and the numerical results studied on a finite lattice in the context of the Dirac sheet configurations.

hep-lat↗
arXiv2004

Recombination of dyons into calorons in SU(2) lattice fields at low temperatures

By cooling of equilibrium lattice fields at finite temperature in SU(2) gauge theory it has been shown that topological objects (calorons) observed on the lattice in the confined phase possess a dyonic substructure which becomes visible under certain circumstances. Here we show that, with decreasing temperature of the equilibrium ensemble, the distribution in the caloron parameter space is modified such that the calorons appear non-dissociated into constituent dyons. Still the calorons have nontrivial holonomy which is demonstrated by the Polyakov line behaviour for these configurations. At vanishing temperature (on a symmetric lattice) topological lumps obtained by cooling possess rotational symmetry in 4D and a characteristic double peak structure of Polyakov lines (defined with respect to temporal and spatial directions) with non-trivial asymptotics.

hep-lat↗
arXiv2004

On Dirac sheet configurations of SU(2) lattice fields

Finite temperature Euclidean SU(2) lattice gauge fields close to the deconfinement phase transition are subjected to cooling. We find relatively stable or absolutely stable configurations with an action below the one-instanton action $S_{inst}=2π^2$ both in the deconfinement and the confinement phases. In this paper we attempt an interpretation of these lowest action configurations. Their action is purely magnetic and amounts to $S/S_{inst} \approx N_t/N_s$, where $N_t$ ($N_s$) is the timelike (spacelike) lattice size, while the topological charge vanishes. In the confined phase part of the corresponding lattice configurations turns out to be absolutely stable with respect to the cooling process in which case Abelian projection reveals a homogeneous, purely Abelian magnetic field closed over the "boundary" in one of the spatial directions. Referring to the dyonic structure established for the confinement phase near $T_c$ and based on the observation made for this phase that such events below the instanton action $S_{inst}$ emerge from dyon-antidyon annihilation, the question of stability (metastability) is discussed for both phases. The hypothetically different dyonic structure of the deconfinement phase, inaccessible by cooling, could explain the metastability.

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