Publications (92)165.98 Total impact
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ABSTRACT: The Boseghost propagator has been proposed as a carrier of the confining force in YangMills theories in minimal Landau gauge. We present the first numerical evaluation of this propagator, using lattice simulations for the SU(2) gauge group in the scaling region. Our data are well described by a simple fitting function, which is compatible with an infraredenhanced Boseghost propagator. This function can also be related to a massive gluon propagator in combination with an infraredfree (FaddeevPopov) ghost propagator. Since the Boseghost propagator can be written as the vacuum expectation value of a BRSTexact quantity and should therefore vanish in a BRSTinvariant theory, our results provide the first numerical manifestation of BRSTsymmetry breaking due to restriction of gaugeconfiguration space to the Gribov region.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The Boseghost propagator has been proposed as a carrier of the confining force in YangMills theories in minimal Landau gauge. We present the first numerical evaluation of this propagator, using lattice simulations for the SU(2) gauge group in the scaling region. Our data are well described by a simple fitting function, which is compatible with an infraredenhanced Boseghost propagator. This function can also be related to a massive gluon propagator in combination with an infraredfree (FaddeevPopov) ghost propagator. Since the Boseghost propagator can be written as the vacuum expectation value of a BRSTexact quantity and should therefore vanish in a BRSTinvariant theory, our results provide the first numerical manifestation of BRSTsymmetry breaking due to restriction of gaugeconfiguration space to the Gribov region.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We address several aspects of gluon propagation at zero and finite temperature. In particular, we study the violation of spectral positivity, we discuss a method to extract the K\"all\'{e}nLehmann spectral density of a particle (be it elementary or bound state) propagator and apply it to compute gluon spectral densities from lattice data. Furthermore, we also consider the interpretation of the Landau gauge gluon propagator at finite temperature as a massive type bosonic propagator.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: A Padé approximation approach, rooted in an infrared moment technique, is employed to provide mass estimates for various glueball states in pure gauge theories. The main input in this analysis are theoretically wellmotivated fits to lattice gluon propagator data, which are by now available for both SU(2)SU(2) and SU(3)SU(3) in 3 and 4 space–time dimensions. We construct appropriate gauge invariant and Lorentz covariant operators in the (pseudo)scalar and (pseudo)tensor sector. Our estimates compare reasonably well with a variety of lattice sources directly aimed at extracting glueball masses.Physics Letters B. 01/2014; 732:247–254.  [Show abstract] [Hide abstract]
ABSTRACT: We consider the problem of "measuring" the K\"all\'enLehmann spectral density of a particle (be it elementary or bound state) propagator by means of 4d lattice data. As the latter are obtained from operations at (Euclidean momentum squared) p^2>=0, we are facing the generically illposed problem of converting a limited data set over the positive real axis to an integral representation, extending over the whole complex p^2plane. We employ a linear regularization strategy, commonly known as the Tikhonov method with Morozov discrepancy principle, with suitable adaptations to realistic data, e.g. with unknown threshold. An important virtue over the (standard) maximum entropy method is the possibility to also probe unphysical spectral densities, as, for example, of a confined gluon. We apply our proposal here to "physical" mock spectral data as a litmus test and then to the lattice SU(3) Landau gauge gluon at zero temperature.10/2013; 89(1).  [Show abstract] [Hide abstract]
ABSTRACT: An infrared moment technique, recently developed by some of us, is employed to provide mass estimates for various glueball states in pure gauge theories. The main input in this analysis are theoretically wellmotivated fits to lattice gluon propagator data, which are by now available for both SU(2) and SU(3) in 3 and 4 spacetime dimensions. We construct appropriate gauge invariant and Lorentz covariant operators in the (pseudo)scalar and (pseudo)tensor sector. Our final numbers compare reasonably well with a variety of lattice sources directly aimed at extracting glueball masses.10/2013;  [Show abstract] [Hide abstract]
ABSTRACT: In this followup paper of 1105.2217 we further discuss the occurrence of a magnetically induced tachyonic instability of the rho meson in the twoflavour SakaiSugimoto 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 nonAbelian DiracBornInfeld (DBI) action. This leads to an extra mass generating effect for the charged heavylight 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 DBIaction. 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.09/2013;  [Show abstract] [Hide abstract]
ABSTRACT: It is well accepted that dealing with the Gribov ambiguity has a major impact on correlation functions in gaugefixed YangMills theories, in particular in the low momentum regime where standard perturbation theory based on the FaddeevPopov approach fails. Recent results, derived from functional tools (DysonSchwinger equations or exact RG) or the effective GribovZwanziger action method, pointed towards e.g. gauge boson correlation functions that are not compatible with the properties of observable degrees of freedom. Although such an observation is a welcome feature for gauge theories exhibiting confinement, it would be a discomfort for gauge theories supplemented with Higgs fields, cfr. the experimental success of the electroweak model based on a SU(2) x U(1) gauge group. The purpose of this short note is to assure that the effective action resolution to the Gribov ambiguity reduces to the standard FaddeevPopov method in the perturbative regime of sufficiently small coupling/large Higgs condensate, thereby not compromising the physical particle spectrum of massive gauge bosons and a massless photon for the SU(2) x U(1) gaugeHiggs model. The closer the theory gets to the limit of vanishing Higgs condensate, the more the Gribov problem resurfaces with all its consequences. We give some speculations w.r.t. the FradkinShenker insights about the phase diagram.Annals of Physics. 09/2013; 
Article: Spontaneous breaking of the BRST symmetry in presence of the Gribov horizon: renormalizability
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ABSTRACT: An all orders algebraic proof of the multiplicative renormalizability of the novel formulation of the GribovZwanziger action proposed in Phys. Rev. D 86, 045005 (2012) [arXiv:1205.3934], and allowing for an exact but spontaneously broken BRST symmetry, is provided.Annals of Physics 06/2013; · 3.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the structure of the gauge propagators of a 3d version of the electroweak interaction in terms of the Higgs vacuum expectation value nu, of the nonAbelian gauge coupling g, and of the Abelian gauge coupling g', when nonperturbative effects related to the nonAbelian gauge fixing are introduced by means of an adapted path integral measure. In the perturbative regime of small nonAbelian coupling g and sufficiently large nu, the wellknown standard Z and W propagators are recovered, together with a massless photon. In general, depending on the relative magnitudes of g, g' and nu, we uncover a quite different propagator structure. In a later stage of research, the results here derived can be used to study the associated phase diagram in more depth.05/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We introduce an effective quark model that is in principle dynamically derivable from the QCD action. An important feature is the incorporation of spontaneous chiral symmetry breaking in a renormalizable fashion. The quark propagator in the condensed vacuum exhibits complex conjugate poles, indicative of an unphysical spectral form, i.e. confined quarks. Moreover, the ensuing mass function can be fitted well to existing lattice data. To validate the physical nature of the new model, we identify not only a massless pseudoscalar (i.e. a pion) in the chiral limit, but we also present reasonable estimates for the rho meson mass and decay constant, employing a contact point interaction and a large N argument to simplify the diagrammatic spectral analysis. We stress that we do not use any experimental input to obtain our numbers, but only rely on our model and lattice quark data.03/2013;  [Show abstract] [Hide abstract]
ABSTRACT: During heavy ion collisions, high temperatures and strong magnetic fields are generated. We employ the gaugegravity duality to study the N_f=2 QCD phase diagram under these extreme conditions in the quenched approximation, in particular we use the nonantipodal SakaiSugimoto model (SSM). We take the different coupling of up and down flavours to the magnetic field into account geometrically, resulting in a split of the chiral phase transition according to flavour. We discuss the influence of the magnetic field on the chiral temperatures in physical GeV units in terms of the choice of the confinement scale in the model, extending hereby our elsewhere presented discussion of fixing the nonantipodal SSM parameters to the deconfinement phase. The flavourdependent (T,L,eB) phase diagram, with variable asymptotic braneantibrane separation L, is also presented, as a direct generalization of the known (T,L) phase diagram of the nonantipodal SSM at zero magnetic field. In particular, for sufficiently small L we are probing a NJLlike boundary field theory in which case we do find results very reminiscent of the predictions in NJL models.Physical review D: Particles and fields 03/2013; 87(10). 
Article: An allorder proof of the equivalence between Gribovʼs nopole and Zwanzigerʼs horizon conditions
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ABSTRACT: The quantization of nonAbelian gauge theories is known to be plagued by Gribov copies. Typical examples are the copies related to zero modes of the Faddeev–Popov operator, which give rise to singularities in the ghost propagator. In this work we present an exact and compact expression for the ghost propagator as a function of external gauge fields, in SU(N)SU(N) Yang–Mills theory in the Landau gauge. It is shown, to all orders, that the condition for the ghost propagator not to have a pole, the socalled Gribovʼs nopole condition, can be implemented by demanding a nonvanishing expectation value for a functional of the gauge fields that turns out to be Zwanzigerʼs horizon function. The action allowing to implement this condition is the Gribov–Zwanziger action. This establishes in a precise way the equivalence between Gribovʼs nopole condition and Zwanzigerʼs horizon condition.Physics Letters B 02/2013; 719(s 4–5):448–453. · 4.57 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this proceeding, we explain a few steps for an alternative extraction of the spectral density of a twopoint function (propagator) based on a discrete set of data points. We present a socalled Tikhonov regularization of this particular inverse problem. We test it on 2 cases: lattice 0++} glueball data and mock gluon data.01/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We present an analytical study of continuum 4d SU(2) gauge Higgs models with a single Higgs field with fixed length in either the fundamental or adjoint representation. We aim at probing the renowned predictions of Fradkin & Shenker on the phase diagram in terms of confinement versus Higgs behaviour, obtained in lattice numerical simulations. We work in the Landau version of the 't Hooft R_\xi gauges in which case we can access potential nonperturbative physics related to the existence of the Gribov copies. In the fundamental case, we clearly show that in the perturbative regime of small gauge coupling constant g and large Higgs vacuum expectation value v, there is a Higgs phase with Yukawa gauge boson propagators without Gribov effects. For a small value of the Higgs vev v and/or large g, we enter a region with Gribov type propagators that have no physical particle interpretation: the gauge bosons are as such confined. The transition between both behaviours is found to be continuous. In the adjoint case, we find evidence of a more drastic transition between the different behaviours for the propagator of the offdiagonal gauge bosons, whereas the "photon", i.e. the diagonal component of the gauge field, is always confined, displaying a propagator of the Gribov type. These findings are in qualitative agreement with those of Fradkin & Shenker as well as with more recent numerical lattice simulations of the fundamental Higgs model. We also carefully discuss in which region of the parameter space (v,g) our approximations are trustworthy.Physical Review D 12/2012; · 4.69 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Either by solving the ghost propagator DSE or through a oneloop computation in the RGZ (Refined GribovZwanziger) formalism, we show that a nontrivial ghostgluon vertex is anyhow required to obtain a ghost propagator prediction compatible with the available corresponding lattice data in the SU(3) case. For the necessary gluon propagator input, we present RGZ tree level fits which account well for the gluon lattice data. Interestingly, this propagator can be rewritten in terms of a running gluon mass. A comparison of both DSE and RGZ results for the ghost propagator is furthermore provided. We also briefly discuss the connection between the RGZ and the OPE $d=2$ gluon condensate.Physical review D: Particles and fields 11/2012; 86(10).  [Show abstract] [Hide abstract]
ABSTRACT: The propagator of a physical degree of freedom ought to obey a K\"{a}ll\'{e}nLehmann spectral representation, with positive spectral density. The latter quantity is directly related to a cross section based on the optical theorem. The spectral density is a crucial ingredient of a quantum field theory with elementary and bound states, with a direct experimental connection as the masses of the excitations reflect themselves into (continuum) $\delta$singularities. In usual lattice simulational approaches to the QCD spectrum the spectral density itself is not accessed. The (bound state) masses are extracted from the asymptotic exponential decay of the twopoint function. Given the importance of the spectral density, each nonperturbative continuum approach to QCD should be able to adequately describe it or to take into proper account. In this work, we wish to present a first trial in extracting an estimate for the scalar glueball spectral density in SU(3) gluodynamics using lattice gauge theory.10/2012;  [Show abstract] [Hide abstract]
ABSTRACT: We study SU(2) threedimensional YangMills theories in presence of Higgs fields in the light of the Gribov phenomenon. By restricting the domain of integration in the functional integral to the first Gribov horizon, we are able to discuss a kind of transition between the Higgs and the confining phase in a semiclassical approximation. Both adjoint and fundamental representation for the Higgs field are considered, leading to a different phase structure.European Physical Journal C 10/2012; 73(3). · 5.25 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study a toy model for an interacting scalar field theory in which the fundamental excitations are confined in the sense of having unphysical, positivityviolating propagators, a fact tracing back to a decomposition of these in propagators with complex conjugate mass poles (the socalled $i$particles). Similar twopoint functions show up in certain approaches to gluon or quark propagators in YangMills gauge theories. We investigate the spectrum of our model and show that suitable composite operators may be constructed having a welldefined K\"all\'{e}nLehmann spectral representation, thus allowing for a particle interpretation. These physical excitations would correspond to the "mesons" of the model, the latter being bound states of two unphysical $i$particles. The meson mass is explicitly estimated from the pole emerging in a resummed class of diagrams. The main purpose of this paper is thus to explicitly verify how a real mass pole can and does emerge out of constituent $i$particles that have complex masses.International Journal of Modern Physics A 08/2012; 28(10). · 1.13 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present an analytic description of numerical results for the Landaugauge SU(2) gluon propagator D(p2), obtained from lattice simulations (in the scaling region) for the largest lattice sizes to date, in d=2, 3 and 4 spacetime dimensions. Fits to the gluon data in 3d and in 4d show very good agreement with the treelevel prediction of the refined GribovZwanziger (RGZ) framework, supporting a massive behavior for D(p2) in the infrared limit. In particular, we investigate the propagator’s pole structure and provide estimates of the dynamical mass scales that can be associated with dimensiontwo condensates in the theory. In the 2d case, fitting the data requires a noninteger power of the momentum p in the numerator of the expression for D(p2). In this case, an infinitevolumelimit extrapolation gives D(0)=0. Our analysis suggests that this result is related to a particular symmetry in the complexpole structure of the propagator and not to purely imaginary poles, as would be expected in the original GribovZwanziger scenario.Physical review D: Particles and fields 05/2012; 85(9).
Publication Stats
1k  Citations  
165.98  Total Impact Points  
Top Journals
Institutions

2002–2014

Ghent University
 Department of Physics and Astronomy
Gand, Flanders, Belgium


2013

University of Coimbra
 Center for Computational Physics
Coímbra, Coimbra, Portugal


2008–2009

Massachusetts Institute of Technology
 Center for Theoretical Physics
Cambridge, Massachusetts, United States


2004

University of Liverpool
 Department of Mathematical Sciences
Liverpool, ENG, United Kingdom
