arXiv subjects

Kurt Langfeld

Publications and source records attributed to Kurt Langfeld.

At least 37 records · Page 2Linked to original sources

arXiv2013

Towards a density of states approach for dense matter systems

The density-of-states method (Phys.Rev.Lett. 109 (2012) 111601) features an exponential error suppression and is not restricted to theories with positive probabilistic weight. It is applied to the SU(2) gauge theory at finite densities of heavy quarks. The key ingredient here is the Polyakov line probability distribution, which is obtained of over 80 orders of magnitude. We briefly address whether the exponential error suppression could be sufficient to simulate theories with a strong sign problem.

hep-lat↗

arXiv2013

Two-colour QCD with heavy quarks at finite densities

We extend the density-of-states approach to gauge systems (LLR method) to QCD at finite temperature and density with heavy quarks. The approach features an exponential error suppression and yields the Polyakov loop probability distribution function over a range of more than hundred orders of magnitude. SU(2) gauge theory is considered in the confinement and the high-temperature phase, and at finite densities of heavy quarks. In the latter case, a smooth rise of the density with the chemical potential is observed, and no critical phenomenon associated with deconfinement due to finite densities is found.

hep-lat↗

arXiv2013

Effective Polyakov line action from strong lattice couplings to the deconfinement transition

We calculate the effective Polyakov line action corresponding to SU(2) lattice gauge theory on a 16^3 X 4 lattice via the "relative weights" method. We consider a variety of lattice couplings, ranging from beta=1.2 in the strong-coupling domain, to beta=2.3 at the deconfinement transition, in order to study how the effective action evolves with beta. Comparison of Polyakov line correlators computed in the effective theory and the underlying gauge theory is used to test the validity of the effective action for beta > 1.4, while for beta=1.2, 1.4 we can compare our effective action to the one obtained from a low-order strong-coupling expansion. Very good agreement is found at all couplings. We find that the effective action is given by a simple expression bilinear in the Polyakov lines. The range of the bilinear term, away from strong coupling, grows rapidly in lattice units as beta increases.

hep-lat↗

arXiv2012

The density of states in gauge theories

The density of states is calculated for a SU(2) and a compact U(1) lattice gauge theory using a modified version of the Wang-Landau algorithm. We find that the density of states of the SU(2) gauge theory can be reliably calculated over a range of 120,000 orders of magnitude for lattice sizes as big as 20^4. We demonstrate the potential of the algorithm by reproducing the SU(2) average action, its specific heat and the critical couplings of the weak first order transition in U(1).

hep-lat↗

arXiv2011

Dense but confined matter as a new state in QCD

Centre sector transitions in QCD-like theories with dynamical quark matter are investigated. In the hadronic phase, these transitions still take place in the infinite volume at zero temperature limit despite of the explicit breaking of the centre symmetry by the matter fields. This finding is supported by simulations of the SU(2) Yang-Mills theory with Higgs matter. The phenomenological impact of the centre sector transitions for dense but confined matter is explained: centre dressed quarks acquire Bose statistics and form a so-called Fermi-Einstein condensate. This mechanism is further illustrated in the Schwinger model where it solves the Silver-Blaze problem.

hep-lat↗

arXiv2011

From confinement to new states of dense QCD matter

Transitions between centre sectors are related to confinement in pure Yang-Mills theories. We study the impact of these transitions in QCD-like theories for which centre symmetry is explicitly broken by the presence of matter. For low temperatures, we provide numerical evidence that centre transitions do occur with matter merely providing a bias towards the trivial centre sector until centre symmetry is spontaneously broken at high temperatures. The phenomenological consequences of these transitions for dense hadron matter are illustrated in an SU(3) effective quark theory: centre dressed quarks undergo condensation due to Bose-type statistics forming a hitherto unknown state of dense but cold quark matter.

hep-lat↗

arXiv2011

Fermi-Einstein condensation in dense QCD-like theories

While pure Yang-Mills theory feature the centre symmetry, this symmetry is explicitly broken by the presence of dynamical matter. We study the impact of the centre symmetry in such QCD-like theories. In the analytically solvable Schwinger model, centre transitions take place even under extreme conditions, temperature and/or density, and we show that they are key to the solution of the Silver-Blaze problem. We then develop an effective SU(3) quark model which confines quarks by virtue of centre sector transitions. The phase diagram by confinement is obtained as a function of the temperature and the chemical potential. We show that at low temperatures and intermediate values for the chemical potential the centre dressed quarks undergo condensation due to Bose like statistics. This is the Fermi Einstein condensation. To corroborate the existence of centre sector transitions in gauge theories with matter, we study (at vanishing chemical potential) the interface tension in the three-dimensional Z2 gauge theory with Ising matter, the distribution of the Polyakov line in the four-dimensional SU(2)-Higgs model and devise a new type of order parameter which is designed to detect centre sector transitions. Our analytical and numerical findings lead us to conjecture a new state of cold, but dense matter in the hadronic phase for which Fermi Einstein condensation is realised.

hep-lat↗

arXiv2010

Yang-Mills streamlines and semi-classical confinement

Semi-classical configurations in Yang-Mills theory have been derived from lattice Monte Carlo configurations using a recently proposed constrained cooling technique which is designed to preserve every Polyakov line (at any point in space-time in any direction). Consequently, confinement was found sustained by the ensemble of semi-classical configurations. The existence of gluonic and fermionic near-to-zero modes was demonstrated as a precondition for a possible semi-classical expansion around the cooled configurations as well as providing the gapless spectrum of the Dirac operator necessary for chiral symmetry breaking. The cluster structure of topological charge of the semi-classical streamline configurations was analysed and shown to support the axial anomaly of the right size, although the structure differs from the instanton gas or liquid. Here, we present further details on the space-time structure and the time evolution of the streamline configurations.

hep-lat↗

arXiv2010

Confinement from semiclassical gluon fields in SU(2) gauge theory

The infrared structure of SU(2) Yang-Mills theory is studied by means of lattice gauge simulations using a new constrained cooling technique. This method reduces the action while all Polyakov lines on the lattice remain unchanged. In contrast to unconstrained cooling, quark confinement is still intact. A study of the Hessian of the Yang-Mills action shows that low action (semi-) classical configurations can be achieved, with a characteristic splitting between collective modes and higher momentum modes. Besides confinement, the semiclassical configurations also support the topological susceptibility and generate spontaneous breakdown of chiral symmetry.We show that they possess a cluster structure of locally mainly (anti-) selfdual objects. By contrast to an instanton or a meron medium, the topological charge of individual clusters is smoothly distributed.

hep-lat↗

arXiv2010

Role of center vortices in chiral symmetry breaking in SU(3) gauge theory

We study the behavior of the AsqTad quark propagator in Landau gauge on SU(3) Yang-Mills gauge configurations under the removal of center vortices. In SU(2) gauge theory, center vortices have been observed to generate chiral symmetry breaking and dominate the infrared behavior of the quark propagator. In contrast, we report a weak dependence on the vortex content of the gauge configurations, including the survival of dynamical mass generation on configurations with vanishing string tension.

hep-lat↗

arXiv2010

A fresh look at the confinement mechanism

Topological configurations, monopoles and vortices, successfully describe quark confinement and the spontaneous breakdown of chiral symmetry. Despite their infinite action, these configurations are relevant due to a subtle cancellation between action and entropy. A natural explanation for this intrinsic fine-tuning is that smooth low action configurations exist which confine and which appear as singular topological objects in certain gauges. To reveal these confining semi-classical configurations, a new cooling method is proposed which largely reduces the action while preserving the asymptotic quark-antiquark potential. First numerical results for a SU(2) gauge theory show that confining configurations with an average plaquette as high as 0.95 do exist.

hep-lat↗

arXiv2010

Confinement and the quark Fermi-surface in SU(2N) QCD-like theories

Yang-Mills theories with a gauge group SU(N_c\=3)and quark matter in the fundamental representation share many properties with the theory of strong interactions, QCD with N_c=3. We show that, for N_c even and in the confinement phase, the gluonic average of the quark determinant is independent of the boundary conditions, periodic or anti-periodic ones. We then argue that a Fermi sphere of quarks can only exist under extreme conditions when the centre symmetry is spontaneously broken and colour is liberated. Our findings are supported by lattice gauge simulations for N_c=2...5 and are illustrated by means of a simple quark model.

hep-lat↗

arXiv2009

Worldline Monte Carlo for fermion models at large N_f

Strongly-coupled fermionic systems can support a variety of low-energy phenomena, giving rise to collective condensation, symmetry breaking and a rich phase structure. We explore the potential of worldline Monte Carlo methods for analyzing the effective action of fermionic systems at large flavor number N_f, using the Gross-Neveu model as an example. Since the worldline Monte Carlo approach does not require a discretized spacetime, fermion doubling problems are absent, and chiral symmetry can manifestly be maintained. As a particular advantage, fluctuations in general inhomogeneous condensates can conveniently be dealt with analytically or numerically, while the renormalization can always be uniquely performed analytically. We also critically examine the limitations of a straightforward implementation of the algorithms, identifying potential convergence problems in the presence of fermionic zero modes as well as in the high-density region.

hep-th↗

arXiv2008

Solution of the Gribov problem from gauge invariance

A new approach to gauge fixed Yang-Mills theory is derived using the Polyakov-Susskind projection techniques to build gauge invariant states. In our approach, in contrast to the Faddeev-Popov method, the Gribov problem does not prevent the gauge group from being factored out of the partition function. Lattice gauge theory is used to illustrate the method via a calculation of the static quark-antiquark potential generated by the gauge fields in the fundamental modular region of Coulomb gauge.

hep-lat↗

arXiv2008

Center vortices and the quark propagator in SU(2) gauge theory

We study the behavior of the AsqTad quark propagator in Landau gauge on quenched SU(2) gauge configurations under the removal of center vortices. In contrast to recent results in SU(3), we clearly see the infrared enhancement of the mass function disappear if center vortices are removed, a sign of the intimate relation between center vortices and chiral symmetry breaking in SU(2) gauge-field theory. These results provide a benchmark with which to interpret the SU(3) results. In addition, we consider vortex-only configurations. On those, the quark dressing function behaves roughly as on the full configurations, and the mass function picks up an almost linear momentum dependence.

hep-lat↗

arXiv2008

The ice-limit of Coulomb gauge Yang-Mills theory

In this paper we describe gauge invariant multi-quark states generalising the path integral framework developed by Parrinello, Jona-Lasinio and Zwanziger to amend the Faddeev-Popov approach. This allows us to produce states such that, in a limit which we call the ice-limit, fermions are dressed with glue exclusively from the fundamental modular region associated with Coulomb gauge. The limit can be taken analytically without difficulties, avoiding the Gribov problem. This is llustrated by an unambiguous construction of gauge invariant mesonic states for which we simulate the static quark--antiquark potential.

hep-lat↗

arXiv2008

Preconditioning Maximal Center Gauge with Stout Link Smearing in SU(3)

Center vortices are studied in SU(3) gauge theory using Maximal Center Gauge (MCG) fixing. Stout link smearing and over-improved stout link smearing are used to construct a preconditioning gauge field transformation, applied to the original gauge field before fixing to MCG. We find that preconditioning successfully achieves higher gauge fixing maxima. We observe a reduction in the number of identified vortices when preconditioning is used, and also a reduction in the vortex-only string tension.

hep-lat↗

arXiv2008

Is the ground state of Yang-Mills theory Coulombic?

We study trial states modelling the heavy quark-antiquark ground state in SU(2) Yang-Mills theory. A state describing the flux tube between quarks as a thin string of glue is found to be a poor description of the continuum ground state; the infinitesimal thickness of the string leads to UV artifacts which suppress the overlap with the ground state. Contrastingly, a state which surrounds the quarks with non-abelian Coulomb fields is found to have a good overlap with the ground state for all charge separations. In fact, the overlap increases as the lattice regulator is removed. This opens up the possibility that the Coulomb state is the true ground state in the continuum limit.

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