478 research outputs found
't Hooft-Polyakov monopoles in lattice SU(N)+adjoint Higgs theory
We investigate twisted C-periodic boundary conditions in SU(N) gauge field
theory with an adjoint Higgs field. We show that with a suitable twist for even
N one can impose a non-zero magnetic charge relative to residual U(1) gauge
groups in the broken phase, thereby creating a 't Hooft-Polyakov magnetic
monopole. This makes it possible to use lattice Monte-Carlo simulations to
study the properties of these monopoles in the quantum theory.Comment: 15 pages, 6 figure
Extended Double Lattice BRST, Curci-Ferrari Mass and the Neuberger Problem
We present Extended Double BRST on the lattice and extend the Neuberger
problem to include the ghost/anti-ghost symmetric formulation of the non-linear
covariant Curci-Ferrari (CF) gauges. We then show how a CF mass regulates the
0/0 indeterminate form of physical observables, as observed by Neuberger, and
discuss the gauge-parameter and mass dependence of the model.Comment: Prepared for 7th Conference on Quark Confinement and the Hadron
Spectrum, Ponta Delgada, Azores, Portugal, 2-7 Sep 2006. 3p
Comparing SU(2) to SU(3) gluodynamics on large lattices
We study the SU(2) gluon and ghost propagators in Landau gauge on lattices up
to a size of 112^4. A comparison with the SU(3) case is made and finite-volume
effects are then investigated. We find that for a large range of momenta the
SU(2) and SU(3) propagators are remarkably alike. In the low-momentum region we
compare with recent results obtained in DSE studies on a 4-torus.Comment: 7 pages, 5 figures, poster presented at the XXV International
Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg,
German
Infrared Behaviour and Running Couplings in Interpolating Gauges in QCD
We consider the class of gauges that interpolates between Landau- and
Coulomb-gauge QCD, and show the non-renormalisation of the two independent
ghost-gluon vertices. This implies the existence of two RG-invariant running
couplings, one of which is interpreted as an RG-invariant gauge parameter. We
also present the asymptotic infrared limit of solutions of the Dyson-Schwinger
equations in interpolating gauges. The infrared critical exponents of these
solutions as well as the resulting infrared fixed point of one of the couplings
are independent of the gauge parameter. This coupling also has a fixed point in
the Coulomb gauge limit and constitutes a second invariant charge besides the
well known colour-Coulomb potential.Comment: 8 pages, 2 figures; v2: minor changes, version published in PR
On dynamical gluon mass generation
The effective gluon propagator constructed with the pinch technique is
governed by a Schwinger-Dyson equation with special structure and gauge
properties, that can be deduced from the correspondence with the background
field method. Most importantly the non-perturbative gluon self-energy is
transverse order-by-order in the dressed loop expansion, and separately for
gluonic and ghost contributions, a property which allows for a meanigfull
truncation. A linearized version of the truncated Schwinger-Dyson equation is
derived, using a vertex that satisfies the required Ward identity and contains
massless poles. The resulting integral equation, subject to a properly
regularized constraint, is solved numerically, and the main features of the
solutions are briefly discussed.Comment: Special Article - QNP2006: 4th International Conference on Quarks and
Nuclear Physics, Madrid, Spain, 5-10 June 200
Infrared exponents and the strong-coupling limit in lattice Landau gauge
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
. 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
. 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.Comment: 17 pages, 12 figures. Revised version (mainly sections I and II);
references and comments on subsequent work on the subject added
Temperature Dependence of Gluon and Ghost Propagators in Landau-Gauge Yang-Mills Theory below the Phase Transition
The Dyson-Schwinger equations of Landau-gauge Yang-Mills theory for the gluon
and ghost propagators are investigated. Numerical results are obtained within a
truncation scheme which has proven to be successful at vanishing temperature.
For temperatures up to 250 MeV we find only minor quantitative changes in the
infrared behaviour of the gluon and ghost propagators. The effective action
calculated from these propagators is temperature-independent within the
numerical uncertainty.Comment: 9 pages, 14 figures, submitted to EPJ C, typos corrected, reference
and 2 minor clarifications added, in v3: one paragraph extended, some
references added, version to appear in EPJ
QCD Propagators at non-vanishing temperatures
We investigate the behaviour of the gluon and ghost propagators, especially
their infrared properties, at non-vanishing temperatures. To this end we solve
their Dyson-Schwinger equations on a torus and find an infrared enhanced ghost
propagator and an infrared vanishing gluon propagator.Comment: 2 pages, 2 figures; talk given by B.G. at the Erice summer school on
Nuclear Physics, Sept. 16 -- 24, 2003, Erice, Ital
Lifshitz and Excited State Quantum Phase Transitions in Microwave Dirac Billiards
We present experimental results for the density of states (DOS) of a
superconducting microwave Dirac billiard which serves as an idealized model for
the electronic properties of graphene. The DOS exhibits two sharp peaks which
evolve into van Hove singularities with increasing system size. They divide the
band structure into regions governed by the \emph{relativistic} Dirac equation
and by the \emph{non-relativistic} Schr\"odinger equation, respectively. We
demonstrate that in the thermodynamic limit a topological transition appears as
a neck-disrupting Lifshitz transition in the number susceptibility and as an
excited state transition in the electronic excitations. Furthermore, we recover
the finite-size scaling typical for excited state quantum phase transitions
involving logarithmic divergences and identify a quasi-order parameter
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