161 research outputs found
Gauge copies in the Landau-DeWitt gauge : a background invariant restriction
The Landau background gauge, also known as the Landau-DeWitt gauge, has found renewed interest during the past decade given its usefulness in accessing the confinement-deconfinement transition via the vacuum expectation value of the Polyakov loop, describable via an appropriate background. In this Letter, we revisit this gauge from the viewpoint of it displaying gauge (Gribov) copies. We generalize the Gribov-Zwanziger effective action in a BRST and background invariant way; this action leads to a restriction on the allowed gauge fluctuations, thereby eliminating the infinitesimal background gauge copies. The explicit background invariance of our action is in contrast with earlier attempts to write down and use an effective Gribov-Zwanziger action. It allows to address certain subtleties arising in these earlier works, such as a spontaneous and thus spurious Lorentz symmetry breaking, something which is now averted
Confining gauge theories and holographic entanglement entropy with a magnetic field
We consider the soft wall model for a heuristic holographical modelling of a
confining gauge theory and discuss how the introduction of a (constant)
magnetic field influences the (de)confinement phase structure. We use the
entanglement entropy as a diagnostic tool in terms of the length of an
entangling strip geometry. Due to the anisotropy introduced by the magnetic
field, we find that the results depend on the orientation of the strip relative
to the field. This allows to identify a richer, anisotropic, interplay between
confinement and a magnetic field than possibly can be extracted from a more
standard order parameter as, for example, the Polyakov loop expectation value.Comment: 25 pages, 31 figure
Veneziano ghost, modified gluon propagator, and gauge copies in QCD
In this short note, we come back to the recent proposal put forward by Kharzeev and Levin [Phys. Rev. Lett. 114, 242001 (2015)], in which they phenomenologically couple the nonperturbative Veneziano ghost to the perturbative gluon, leading to a modified gluon propagator (the "glost") of the Gribov type, with complex poles. As such, a possible link was made between the QCD topological theta-vacuum (Veneziano ghost) and color confinement (no physically observable gluons). We discuss some subtleties concerning gauge (Becchi-Rouet-Stora-Tuytin) invariance of this proposal, related to the choice of Feynman gauge. We draw particular attention to the incompatibility in the longitudinal sector with available nonperturbative results for the linear covariant gauge. We furthermore provide an example in the Landau gauge of a similar phenomenological vertex that also describes the necessary Veneziano ghost but does not affect the Landau gauge gluon propagator
Thermal entropy of a quark-antiquark pair above and below deconfinement from a dynamical holographic QCD model
We discuss the entropy carried by a quark-antiquark pair, in particular across the deconfinement transition. We therefore rely on a self-consistent solution to Einstein-Maxwell-dilaton gravity, capable of mimicking essential features of QCD. In particular we introduce a novel model that still captures well the QCD confinement and deconfinement phases, while allowing the introduction of a temperature in a phase which resembles the confined phase, this thanks to it being dual to a small black hole. We pay due attention to some subtleties of such a model. We confirm the lattice picture of a strong build-up of thermal entropy towards the critical temperature T-c, both coming from below or above T-c. We also include a chemical potential, confirming this entropic picture and we consider its effect on the speed of sound. Moreover, the temperature dependent confinement phase from the holography side allows us to find a string tension that does not vanish at T-c, a finding also supported by lattice QCD
A magnetic instability of the non-Abelian Sakai-Sugimoto model
In this follow-up paper of 1105.2217 we further discuss the occurrence of a
magnetically induced tachyonic instability of the rho meson in the two-flavour
Sakai-Sugimoto model, uplifting two remaining approximations in the previous
paper. That is, firstly, the magnetically induced splitting of the branes is
now taken into account, evaluating without approximations the symmetrized trace
which enters in the non-Abelian Dirac-Born-Infeld (DBI) action. This leads to
an extra mass generating effect for the charged heavy-light rho meson through a
holographic Higgs mechanism. Secondly, we compare the results in the
approximation to second order in the field strength to the results using the
full DBI-action. Both improvements cause an increase of the critical magnetic
field for the onset of rho meson condensation. In addition, the stability in
the scalar sector in the presence of the magnetic field is discussed.Comment: 42 pages, v2: version accepted for publication in JHE
Radiation Gauge in AdS/QCD: inadmissibility and implications on spectral functions in the deconfined phase
We point out a subtlety in choosing the radiation gauge (A_z=0 combined with
the Lorenz gauge) for gauge fields in AdS/QCD for black hole backgrounds. We
then demonstrate the effect of this on the momentum-dependence of the spectral
functions of the J/psi vector meson, showing a spreading with momentum and a
breaking of isotropy, in contrast to previous results in the literature. We
also discuss the dependence on a background magnetic field, following our
earlier proposed model.Comment: 10 pages, v2: added reference, version accepted for publicatio
Holographic estimate of heavy quark diffusion in a magnetic field
We study the influence of a background magnetic field on the vector
meson in a DBI-extension of the soft wall model, building upon our earlier work
Phys. Rev. D91, 086002 (2015). In this specific holographic QCD model, we
discuss the heavy quark number susceptibility and diffusion constants of charm
quarks and their dependence on the magnetic field by either a hydrodynamic
expansion or by numerically solving the differential equation. This allows us
to determine the response of these transport coefficients to the magnetic
field. The effects of the latter are considered both from a direct as indirect
(medium) viewpoint. As expected, we find a magnetic field induced anisotropic
diffusion, with a stronger diffusion in the longitudinal direction compared to
the transversal one. We backup, at least qualitatively, our findings with a
hanging string analysis of heavy quark diffusion in a magnetic field. From the
quark number susceptibility we can extract an estimate for the effective
deconfinement temperature in the heavy quark sector, reporting consistency with
the phenomenon of inverse magnetic catalysis.Comment: 27 pages. v2: extra discussions and references, compatible with
version accepted by Phys.Rev.
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