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Search Results: 1 - 10 of 5872 matches for " Lorenz von Smekal "
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Universal Aspects of QCD-like Theories
Lorenz von Smekal
Physics , 2012, DOI: 10.1016/j.nuclphysbps.2012.06.006
Abstract: In these lectures I review some basic examples of how the concepts of universality and scaling can be used to study aspects of the chiral and the deconfinement transition, if not in QCD directly but in QCD-like theories. As an example for flavor dynamics I discuss a quark-hadron model to describe the phase diagram of two-color QCD with the functional renormalization group. Universal aspects of deconfinement are illustrated mainly in the 2+1 dimensional SU(N) gauge theories with second order transition where many exact results from spin models can be exploited.
SU(2) lattice gauge theory in 2+1 dimensions: critical couplings from twisted boundary conditions and universality
Sam Edwards,Lorenz von Smekal
Physics , 2009, DOI: 10.1016/j.physletb.2009.10.063
Abstract: We present a precision determination of the critical coupling beta_c for the deconfinement transition in pure SU(2) gauge theory in 2+1 dimensions. This is possible from universality, by intersecting the center vortex free energy as a function of the lattice coupling beta with the exactly known value of the interface free energy in the 2D Ising model at criticality. Results for lattices with different numbers of sites N_t along the Euclidean time direction are used to determine how beta varies with temperature for a given N_t around the deconfinement transition.
The strong-coupling limit of lattice Landau gauge
Andre Sternbeck,Lorenz von Smekal
Physics , 2008,
Abstract: We report on our recent study of the gluon and ghost propagators of pure SU(2) minimal lattice Landau gauge in the strong-coupling limit. In this limit, we find evidence of the conformal infrared behaviour of these propagators as predicted by functional continuum methods. However, in the strong-coupling limit this happens for lattice momenta with a^2q^2>1, in units of the lattice spacing a. Deviations from conformal scaling for a^2q^2<1 are well parameterised by a transverse gluon mass. A comparison of various lattice definitions of gauge potentials, all equivalent in the continuum limit, shows that (a) both the critical exponent and coupling can be extracted unambiguously from the high-momentum data in the strong-coupling limit, in good agreement with the continuum predictions; but that on the other hand (b) the massive branch depends on the definition of lattice gluon fields and is thus not unambiguously defined. We demonstrate that this ambiguity is also present in the low-momentum region for commonly used values of the lattice coupling in SU(2).
The strong-coupling limit of minimal lattice Landau gauge
Andre Sternbeck,Lorenz von Smekal
Physics , 2008,
Abstract: We study the gluon and ghost propagators of lattice Landau gauge in the strong coupling limit $\beta = 0$ in pure SU(2) lattice gauge theory to find evidence of the conformal infrared behaviour of these propagators as predicted by a variety of functional continuum methods for asymptotically small momenta $q^2 \ll \Lambda_\mathrm{QCD}^2$. In the strong-coupling limit, this same behaviour is obtained for the larger values of $a^2q^2$ (in units of the lattice spacing $a$), where it is otherwise swamped by the gauge field dynamics. Deviations for $a^2 q^2 < 1 $ are well parametrized by a transverse gluon mass $\propto 1/a$. Perhaps unexpectedly, these deviations are thus no finite-volume effect but persist in the infinite-volume limit. They furthermore depend on the definition of gauge fields on the lattice, while the asymptotic conformal behaviour does not.
Infrared exponents and the strong-coupling limit in lattice Landau gauge
Andre Sternbeck,Lorenz von Smekal
Physics , 2008, DOI: 10.1140/epjc/s10052-010-1381-8
Abstract: We study the gluon and ghost propagators of lattice Landau gauge in the strong-coupling limit beta=0 in pure SU(2) lattice gauge theory to find evidence of the conformal infrared behavior of these propagators as predicted by a variety of functional continuum methods for asymptotically small momenta $q^2 \ll \Lambda_\mathrm{QCD}^2$. In the strong-coupling limit, this same behavior is obtained for the larger values of a^2q^2 (in units of the lattice spacing a), where it is otherwise swamped by the gauge field dynamics. Deviations for a^2q^2 < 1 are well parameterized by a transverse gluon mass $\propto 1/a$. Perhaps unexpectedly, these deviations are thus no finite-volume effect but persist in the infinite-volume limit. They furthermore depend on the definition of gauge fields on the lattice, while the asymptotic conformal behavior does not. We also comment on a misinterpretation of our results by Cucchieri and Mendes in Phys. Rev. D81 (2010) 016005.
Lattice BRST without Neuberger 0/0 problem
Lorenz von Smekal,Manon Bischoff
Physics , 2013,
Abstract: We illustrate in a simple toy model how the methods of SUSY quantum mechanics and topological quantum field theory can be used for covariant gauge-fixing with unbroken BRST symmetry on a finite lattice.
The Infrared Behavior of QCD Propagators in Landau Gauge
Reinhard Alkofer,Lorenz von Smekal
Physics , 2000, DOI: 10.1016/S0375-9474(00)00402-4
Abstract: Some features of the solutions to the truncated Dyson-Schwinger equations(DSEs) for the propagators of QCD in Landau gauge are summarized. In particular, the Kugo-Ojima confinement criterion is realized, and positivity of transverse gluons is manifestly violated in these solutions. In Landau gauge, the gluon-ghost vertex function offers a convenient possibility to define a nonperturbative running coupling. The infrared fixed point obtained from this coupling which determines the 2-point interactions of color-octet quark currents implies the existence of unphysical massless states which are necessary to escape the cluster decomposition of colored clusters. The gluon and ghost propagators, and the nonperturbative running coupling, are compared to recent lattice simulations. A significant deviation of the running coupling from the infrared behavior extracted in simulations of 3-point functions is attributed to an inconsistency of asymmetric subtraction schemes due to a consequence of the Kugo-Ojima criterion: infrared enhanced ghosts.
The Infrared Behavior of QCD Green's Functions - Confinement, Dynamical Symmetry Breaking, and Hadrons as Relativistic Bound States
Reinhard Alkofer,Lorenz von Smekal
Physics , 2000, DOI: 10.1016/S0370-1573(01)00010-2
Abstract: Recent studies of QCD Green's functions and their applications in hadronic physics are reviewed. We briefly discuss the issues of gauge fixing, BRS invariance and positivity. Evidence for the violation of positivity by quarks and transverse gluons in the covariant gauge is collected, and it is argued that this is one manifestation of confinement. We summarise the derivation of the Dyson-Schwinger equations (DSEs) of QED and QCD. The influence of instantons on DSEs in a 2-dimensional model is mentioned. Solutions for the Green's functions in QED in 2+1 and 3+1 dimensions provide tests of various schemes to truncate DSEs. We discuss possible extensions to QCD and their limitations. Truncation schemes for DSEs of QCD are discussed in the axial gauge and in the Landau gauge. We review the available results from a systematic non-perturbative expansion scheme established for Landau gauge QCD. Comparisons to related lattice results, where available, are presented. The applications of QCD Green's functions to hadron physics are summarized. Properties of ground state mesons are discussed on the basis of the Bethe-Salpeter equation for quarks and antiquarks. The Goldstone nature of pseudoscalar mesons and mechanisms of diquark confinement are reviewed. We discuss some properties of ground state baryons based on their description as Bethe-Salpeter/Faddeev bound states of quark-diquark correlations in the quantum field theory of confined quarks and gluons.
What the Infrared Behavior of QCD Green Functions can tell us about Confinement in the Covariant Gauge
Lorenz von Smekal,Reinhard Alkofer
Physics , 2000,
Abstract: We review aspects of confinement in the covariant and local description of QCD and discuss to what extend our present knowledge of the infrared behavior of QCD Green functions can support this description. In particular, we emphasize: the positivity violations of transverse gluon and quark states, the Kugo-Ojima confinement criterion, and the conditions necessary to avoid the decomposition property for colored clusters. We summarize how these issues relate to the infrared behavior of the propagators in Landau gauge QCD as extracted from solutions to truncated Dyson-Schwinger equations and lattice simulations.
Monte-Carlo simulation of the tight-binding model of graphene with partially screened Coulomb interactions
Dominik Smith,Lorenz von Smekal
Physics , 2014, DOI: 10.1103/PhysRevB.89.195429
Abstract: We report on Hybrid-Monte-Carlo simulations of the tight-binding model with long-range Coulomb interactions for the electronic properties of graphene. We investigate the spontaneous breaking of sublattice symmetry corresponding to a transition from the semimetal to an antiferromagnetic insulating phase. Our short-range interactions thereby include the partial screening due to electrons in higher energy states from ab initio calculations based on the constrained random phase approximation [T.O.Wehling {\it et al.}, Phys.Rev.Lett.{\bf 106}, 236805 (2011)]. In contrast to a similar previous Monte-Carlo study [M.V.Ulybyshev {\it et al.}, Phys.Rev.Lett.{\bf 111}, 056801 (2013)] we also include a phenomenological model which describes the transition to the unscreened bare Coulomb interactions of graphene at half filling in the long-wavelength limit. Our results show, however, that the critical coupling for the antiferromagnetic Mott transition is largely insensitive to the strength of these long-range Coulomb tails. They hence confirm the prediction that suspended graphene remains in the semimetal phase when a realistic static screening of the Coulomb interactions is included.
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