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The interpretation of the Landau gauge lattice gluon propagator as a massive type bosonic propagator is investigated. Three different scenarios are discussed: i) an infrared constant gluon mass; ii) an ultraviolet constant gluon mass; iii) a momentum dependent mass. We find that the infrared data can be associated with a massive propagator up to momenta $\sim 500$ MeV, with a constant gluon mass of 723(11) MeV, if one excludes the zero momentum gluon propagator from the analysis, or 648(7) MeV, if the zero momentum gluon propagator is included in the data sets. The ultraviolet lattice data is not compatible with a massive type propagator with a constant mass. The scenario of a momentum dependent gluon mass gives a decreasing mass with the momentum, which vanishes in the deep ultraviolet region. Furthermore, we show that the functional forms used to describe the decoupling like solution of the Dyson-Schwinger equations are compatible with the lattice data with similar mass scales., Comment: Version to appear in J. Phys. G. New version include some rewriting and new analysis. In particular, the section on the running mass is new

We consider the gluon propagator $D(p^2)$ at various lattice sizes and spacings in the case of pure SU(3) Yang-Mills gauge theories using the Landau gauge fixing. We discuss a class of fits in the infrared region in order to (in)validate the tree level analytical prediction in terms of the (Refined) Gribov-Zwanziger framework. It turns out that an important role is played by the presence of the widely studied dimension two gluon condensate $\braket{A^2}$. Including this effect allows to obtain an acceptable fit up to 1 \'{a} 1.5 GeV, while corroborating the Refined Gribov-Zwanziger prediction for the gluon propagator. We also discuss the infinite volume extrapolation, leading to the estimate $D(0)=8.3\pm0.5\text{GeV}^{-2}$. As a byproduct, we can also provide the prediction $\braket{g^2 A^2}\approx 3\text{GeV}^2$ obtained at the renormalization scale $\mu=10\text{GeV}$., Comment: 17 pages, 10 figures, updated version, accepted for publication in Phs.Rev.D

he Wu-Yang monopole for pure SU(2) Yang-Mills theory is revisited. New classical solutions with finite energy are found for a generalized Wu-Yang configuration. Our method relies on known asymptotic series solutions and explores the conformal invariance of the theory to set the scale for the monopole configurations. Furthermore, by breaking the scale invariance of the pure Yang-Mills theory by a term which simulates the coupling to quark fields, four different monopole configurations with an energy of 2.9 GeV and spatial extent of 1.2 fm, 1.4 fm, 2.4 fm and 2.6 fm are obtained. These configurations may play a role in the transition to the plasma phase., Comment: The article was considerable expanded, including new sections on numerical solutions, solutions with discontinuous first derivatives and solutions with an explicit breaking of conformal invariance.

In lattice QCD, a confining potential for a static quark-antiquark pair can be computed with the Wilson loop technique. This potential, dominated by a linear potential at moderate distances, is consistent with the confinement with a flux tube, an extended and scalar system also directly observable in lattice QCD. Quantized flux tubes have also been observed in another class of confinement, the magnetic confinement in type II superconductors. On the other hand the solution of Schwinger Dyson Equations, say with the Landau gauge fixing and the truncation of the series of Feynman diagrams, already at the rainbow level for the self energy and at the ladder level for the Bethe Salpeter equation, provides a signal of a possible inverse quartic potential in momentum space derived from one gluon and one ghost exchange, consistent with confinement. Here we address the successes, difficulties and open problems of the matching of these two different perspectives of confinement, the Schwinger-Dyson perspective versus the flux tube perspective., Comment: 12 pages, 18 figures; talk presented at QCD-TNT, Trento, 7-11 sep 2009

The Landau gauge lattice gluon propagator is discussed for different sets of lattices. Particular attention is given to its infrared properties. Our results show that the lattice propagator can be made compatible with either the decoupling-like or the scaling-like solution of the Dyson-Schwinger equations. Furthermore, the analysis of the Cucchieri-Mendes bounds is performed onsidering large volume simulations and the Oliveira-Silva ratios are computed. If the first do not give a clear answer about the value of D(0), the second method favors a $D(0) = 0$. Finally, the SU(3) and SU(2) propagators are compared in the infrared. It comes out that the propagators are different although the infrared exponents seem to be similar. The analysis suggests a scaling behaviour $D(0) \sim N$ with the gauge group SU(N)., Comment: Talk given at International Workshop on QCD Green's Functions, Confinement, and Phenomenology - QCD-TNT09, September 2009, Trento, Italy

We report on the infrared behaviour of the lattice gluon propagator. The bounds on D(0) and their behaviour with the lattice volume are investigated, together with the full propagator. Moreover, a study of the gluon propagator using different lattice spacings is carried out., Comment: 7 pages, 3 figures, talk presented at Lattice 2009 (Vacuum Structure and Confinement)

In this work we calculate the effects of the electroweak flavour changing neutral currents dimension six effective operators on single top production at the LHC. These results are then combined with previous ones we have obtained for the strong sector. This allow us, for the first time, to perform a combined analysis of flavour changing neutral currents in top production with all contributing dimension six operators. Finally, we study the feasibility of their observation and characterization both at the Tevatron and at the LHC., Comment: 16 pages, 13 figures

We study exotic static 3-body potentials, utilizing generalized Wilson Loops in SU(3) lattice QCD. For the quark-antiquark-gluon techniques we address the angles of 0, 45, 60, 90, 120, 135 and 180 degrees, between the quark-gluon and the antiquark-gluon segments. We calculate the form of the static potential and discuss whether, or not, two-body interactions exist between the three different bodies, and study the existence of repulsion between the strings. We also perform a first study of the interactions in the system of three gluon., Comment: 7 pages, 6 figures, Presented at the 26th International Symposium on Lattice Field Theory (Lattice 2008), Williamsburg, Virginia, USA, July 14-19, 2008

The Cucchieri-Mendes bounds for the gluon propagator are discussed for the four dimensional pure gauge SU(3) theory. Assuming a pure power law dependence on the inverse of the lattice volume, the lattice data gives a vanishing zero momentum gluon propagator in the infinite volume limit. Our investigation shows that this result is robust against variations of the lattice volumes and corrections to the power law. Moreover, it is consistent with the Gribov-Zwanziger horizon condition and contradicts the SU(2) analysis, which assumed the same type of dependence on the inverse of the volume. Our analysis considers also more general ansatze that, although not conclusive, open the possibility of having $D(0) \ne 0$ in the infinite volume limit. A solution to this puzzle requires further investigations., Comment: Minor changes. Figures redone using a different axis variable

The Landau-gauge gluon propagator is computed for the SU(3) gauge theory on lattices up to a size of $32^3 \times 200$. We use the standard Wilson action at $\beta = 6.0$ and compare our results with previous computations using large asymmetric and symmetric lattices. In particular, we focus on the impact of the lattice geometry and momentum cuts to achieve compatibility between data from symmetric and asymmetric lattices for a large range of momenta., Comment: Poster presented at Lattice2007, Regensburg, July 30 - August 4, 2007

We compare two Landau gauge fixing methods, aiming to find the global maximum of the gauge fixing functional. Moreover, a systematic effect of Gribov copies in the gluon and ghost propagators computed in Landau gauge is presented and discussed., Comment: 7 pages, 3 figures, poster presented at Lattice 2007 (Vacuum structure and confinement)

The compatibility of the pure power law infrared solution of QCD Dyson-Schwinger equations (DSE) and lattice data for the gluon and ghost propagators in Landau gauge is discussed. For the gluon propagator, the lattice data is compatible with the DSE infrared solution with an exponent $\kappa\sim0.53$, measured using a technique that suppresses finite volume effects and allows to model these corrections to the lattice data. For the ghost propagator, the lattice data does not seem to follow the infrared DSE power law solution., Comment: 7 pages, 3 figures, talk presented at Lattice 2007 (Vacuum structure and confinement)

A comparative study of the lattice Landau gauge gluon and ghost propagators for SU(2) and SU(3) pure Yang-Mills theories is carried out. The data were specially produced with equivalent lattice parameters to allow for a careful comparison of the two cases. We find very good agreement between the two theories. Our results seem to confirm the predicton of Schwinger-Dyson equations that the infrared exponents are independent of the gauge group SU(N)., Comment: Talk presented at Lattice2007, Regensburg, July 30 - August 4, 2007 References corrected and added a new reference

The infrared behavior of gluon and ghost propagators in Yang-Mills theories is of central importance for understanding quark and gluon confinement in QCD. While simulations of pure SU(3) gauge theory correspond to the physical case in the limit of infinite quark mass, the SU(2) case (i.e. pure two-color QCD) is usually employed as a simplification, in the hope that qualitative features be the same as for the SU(3) case. Here we carry out the first comparative study of lattice (Landau) propagators for these two gauge groups. Our data were especially produced with equivalent lattice parameters in order to allow a careful comparison of the two cases. We find very good agreement between SU(2) ans SU(3) propagators, showing that in the IR limit the equivalence of the two cases is quantitative, at least down to about 1 GeV. Our results suggest that the infrared behavior of these propagators is independent of the gauge group SU(N_c), as predicted by Schwinger-Dyson equations., Comment: 5 pages, 1 table, 4 figures; modified title, abstract and text; added references; to appear in Phys. Rev. D

The compatibility of the pure power law infrared solution of QCD and lattice data for the gluon and ghost propagators in Landau gauge is discussed. For the gluon propagator, the lattice data is well described by a pure power law with an infrared exponent $\kappa \sim 0.53$, in the Dyson-Schwinger notation. $\kappa$ is measured using a technique that suppresses finite volume effects. This value implies a vanishing zero momentum gluon propagator, in agreement with the Gribov-Zwanziger confinement scenario. For the ghost propagator, the lattice data seem not to follow a pure power law, at least for the range of momenta accessed in our simulation., Comment: Substantial rewriting with new material included. The new version includes a new and more deep discussion of the finite volume and asymmetry effects

We study the long distance interaction for hybrid hadrons, with a static gluon, a quark and an antiquark with lattice QCD techniques. A Wilson loop adequate to the static hybrid three-body system is developed and, using a 24^3 x 48 periodic lattice with beta=6.2 and a ~ 0.075 fm, two different geometries for the gluon-quark segment and the gluon-antiquark segment are investigated. When these segments are perpendicular, the static potential is compatible with confinement realized with a pair of fundamental strings, one linking the gluon to the quark and another linking the same gluon to the antiquark. When the segments are parallel and superposed, the total string tension is larger and agrees with the Casimir Scaling measured by Bali. This can be interpreted with a type-II superconductor analogy for the confinement in QCD, with repulsion of the fundamental strings and with the string tension of the first topological excitation of the string (the adjoint string) larger than the double of the fundamental string tension., Comment: 4 pages RevTeX, 4 figures

The charmonium and bottomonium spectra computed from a potential defined from a single gauge configuration, obtained from solving the classical field equations, is discussed. The theoretical spectra shows good agreement with the measured states. A discussion of possible interpretations, within the same non-relativistic potential model, for the new charmonia states X(3872), $\chi_{c1} (2P)$ and Y(4260) is performed. In particular, we give predictions for electromagnetic E1 transitions for various scenarios., Comment: Talk given at "Quark Confinement and the Hadron Spectrum VII", Ponta Delgada, Sao Miguel, Azores, Portugal, Sept. 2-7, 2006

We report on the infrared limit of the quenched lattice Landau gauge gluon propagator computed from large asymmetric lattices. In particular, the compatibility of the pure power law infrared solution $(q^2)^{2\kappa}$ of the Dyson-Schwinger equations is investigated and the exponent $\kappa$ is measured. Some results for the ghost propagator and for the running coupling constant will also be shown., Comment: 3 pages, 2 figures. Talk given at "Quark Confinement and the Hadron Spectrum VII", Azores, Portugal, 2-7 September 2006

A generalized Cho-Faddeev-Niemi ansatz for SU(3) Yang-Mills is investigated. The corresponding classical field equations are solved for its simplest parametrization. From these solutions it is possible to define a confining central non-relativistic potential used to study heavy quarkonia. The associated spectra reproduces the experimental spectra with an error of less than 3% for charmonium and 1% for bottomonium. Moreover, the recently discovered new charmonium states can be accomodate in the spectra, keeping the same level of precision. The leptonic width show good agreement with the recent measurements. The charmonium and bottomonium E1 electromagnetic transitions widths are computed and compared with the experimental values., Comment: 3 pages. Talk at QNP06, 5th-10th June, Madrid, Spain

We report on the infrared limit of the quenched lattice Landau gauge gluon and ghost propagators computed from large asymmetric lattices. In particular, the compatibility of the pure power law infrared solutions of the Dyson-Schwinger equations with the lattice data is investigated and the exponent $\kappa$ is measured. The gluon lattice data favour $\kappa \sim 0.52$, which would imply a vanishing zero momentum gluon propagator. For the subset of lattices where the ghost propagator was computed, the data is not compatible with a pure power law. Our data also show a decreasing running coupling in the infrared region. Furthermore, positivity violation for the gluon propagator is also verified., Comment: 7 pages, 5 figures, talk presented at Lattice 2006 (Confinement and Topology); minor changes, PoS(LAT2006)075

We report on the infrared limit of the quenched lattice Landau gauge gluon propagator computed from large asymmetric lattices. In particular, the compatibility of the pure power law infrared solution $(q^2)^{2\kappa}$ of the Dyson-Schwinger equations is investigated and the exponent $\kappa$ is measured. The lattice data favours $\kappa \sim 0.52$, which would imply a vanishing zero momentum gluon propagator as predicted by the Kugo-Ojima confinement mechanism and the Zwanziger horizon condition. Results for the ghost propagator and for the running coupling constant are shown., Comment: 7 pages, 13 figures, proceedings of "Infrared QCD in Rio", Rio de Janeiro, 5-9 June, 2006; minor changes, figure 1 now includes ghost data

We report on the infrared limit of the quenched lattice Landau gauge gluon and ghost propagators as well as the strong coupling constant computed from large asymmetric lattices. The infrared lattice propagators are compared with the pure power law solutions from Dyson-Schwinger equations (DSE). For the gluon propagator, the lattice data is compatible with the DSE solution. The preferred measured gluon exponent being $\sim 0.52$, favouring a null zero momentum propagator. The lattice ghost propagator shows finite volume effects and, for the volumes considered, the propagator does not follow a pure power law. Furthermore, the strong coupling constant is computed and its infrared behaviour investigated., Comment: Talk given at QNP06; final version with improved english, accepted for publication at EPJA

A generalized Faddeev-Niemi ansatz for the gluon field is discussed. In its simplest parametrization, the ansatz allows a solution of the classical SU(3) Yang-Mills equations. From these solutions a confining potential for heavy quarkonia is defined. The investigation of charmonium spectra proves that the potential is able to reproduce the experimental spectra at the level of 3%. Moreover, charmonium leptonic and strong decays are investigated. The results are in fair agreement with experimental figures and are in line with other quark model calculations., Comment: Substantial rewriting

The infrared limit of the lattice Landau gauge gluon propagator is studied. We show that the lattice data is compatible with the pure power law $(q^2)^{2 \kappa}$ solution of the Dyson-Schwinger equations. Using various lattice volumes, the infinite volume limit for the exponent $\kappa$ is measured. Although, the results allow $\kappa = 0.498 - 0.525$, the lattice data favours $\kappa \sim 0.52$, which would imply a vanishing zero momentum gluon propagator., Comment: Final version to appear in Phys Rev D

We perform a model independent analysis of the flavour changing strong interaction vertices relevant to the LHC. In particular, the contribution of dimension six operators to single top production in various production processes is discussed, together with possible hints for identifying signals and setting bounds on physics beyond the standard model., Comment: Authors corrections (references added)

We report on a preliminary study of the volume dependence of the gluon propagator. The propagator is computed using different lattice volumes, its extrapolation to infinite volume is investigated with particular attention to the its IR behaviour. Our data shows a mild but measurable dependence with the volume. Unfortunately, we are not able yet to clarify its behaviour close to zero momentum., Comment: 6 pages, 5 figures. Talk presented at Lattice 2005 (Theoretical Developments), Dublin, July 25-30, 2005

The gluon propagator is computed in large asymmetric lattices, accessing momenta around 100 MeV and smaller. Our study tries to check the compatibility of the recent solutions of the gluon-ghost Dyson-Schwinger equations with lattice results. In particular the exponent $\kappa$, which characterizes the solutions for the infrared, is measured. Results favours a vanishing zero momentum gluon propagator. We also report on the compatibility of various functional forms used to fit the Dyson-Schwinger solution for the full range of momenta and the lattice gluon propagator., Comment: 6 pages, 3 figures. Talk presented at Lattice 2005 (Theoretical Developments), Dublin, July 25-30, 2005

It is showed that potentials derived from classical solutions of the SU(3) Yang-Mills theory can provide confining potentials that reproduce the heavy quarkonium spectrum within the same level of precision as the Cornell potential., Comment: Talk given at Quark Confinement and the Hadron Spectrum VI 2004, 21-25 Sept, Italy

The quenched Landau gauge gluon propagator is investigated in lattice QCD with large assimetric lattices, accessing momenta as low as $q \sim 100$ MeV or smaller. Our investigation focus on the IR limit of the gluon dressing function, testing the compatibility with recent solutions of the Dyson-Schwinger equations. In particular, the low energy parameters $\kappa$ and $\alpha (0)$ are measured., Comment: 3 pages, 1 figure. Talk given at Quark Confinement and the Hadron Spectrum VI 2004, Italy, 21-25 Sep. References correct

We address the problem of Gribov copies in lattice QCD. The gluon propagator is computed, in the Landau gauge, using 302 ($\beta = 5.8$) $12^4$ configurations gauge fixed to different copies. The results of the simulation shows that: i) the effect of Gribov copies is small (less than 10%); ii) Gribov copies change essentially the lowest momenta components ($q < 2.6$ GeV); iii) within the statistical accuracy of our simulation, the effect of Gribov copies is resolved if statistical errors are multiplied by a factor of two or three. Moreover, when modelling the gluon propagator, different sets of Gribov copies produce different sets of parameters not, necessarily, compatible within one standard deviation. Finally, our data supports a gluon propagator which, for large momenta, behaves like a massive gluon propagator with a mass of 1.1 GeV.

An algorithm for gauge fixing to the minimal Landau gauge in lattice QCD is described. The method, a combination of an evolutionary algorithm with a steepest descent method, is able to solve the problem of the nonperturbative gauge fixing. The performance of the combined algorithm is investigated on $8^4$, $\beta = 5.7$, and $16^4$, $\beta = 6.0$, lattice SU(3) gauge configurations., Comment: To appear in Comp. Phys. Comm. Minor changes in text

In this paper we compute classical Minkowsky spacetime solutions of pure SU(2) and SU(3) gauge theories, in Landau gauge. The solutions are regular everywhere except at the origin and/or infinity, are characterized by a four momentum $k$ such that $k^2 = 0$ and resemble QED configurations. The classical solutions suggest a particle-independent description of hadrons, similarly to the Atomic and Nuclear energy levels, which is able to reproduce the heavy quarkonium spectrum with a precision below 10%. Typical errors in the theoretical mass prediction relative to the measured mass being of the order of 2-4%., Comment: New version includes acknowledges

Regular classical solutions of pure SU(3) gauge theories, in Minkowsky spacetime, are computed in the Landau gauge. The classical fields have an intrinsic energy scale and produce quark confinement if interpreted in the sense of a nonrelativistic potential. Moreover, the quark propagator in the background of these fields vanishes at large positive and negative time and space separations., Comment: 10 pages, 1figure

We address the problem of the gauge fixing versus Gribov copies in lattice gauge theories. For the Landau gauge, results show that a suitable combination of evolutionary algorithms with traditional steepest descent methods identifies the global maximum of the optimisation function. We discuss the performance of the combined algorithm on small cubic lattices for SU(2) and SU(3)., Comment: Lattice2001(algorithms)

The problem of identifying the dynamical degrees of freedom for SU(2) gauge theories is discussed. After studying SU(N) theories, it is shown that classical pure SU(2) gauge theory is equivalent to an abelian theory. Finally, we prove that the Maximal Abelian Gauge correctly identifies the abelian degrees of freedom of the SU(2) theory.

We present the results of the first lattice study of semileptonic decays of baryons containing a b-quark. Predictions for the decay distributions are given and the Isgur-Wise functions for heavy baryons are computed, for values of the velocity transfer up to about omega=1.2. The computations are performed on a 24^3 x 48 lattice at beta=6.2 using the Sheikholeslami-Wohlert action in the quenched approximation., Comment: 55 pages, 17 ps figures, Latex 2.09, uses epsf

We present an exploratory study of a one-parameter family of covariant, non-perturbative lattice gauge-fixing conditions, that can be implemented through a simple Monte Carlo algorithm. We demonstrate that at the numerical level the procedure is feasible, and as a first application we examine the gauge dependence of the gluon propagator., Comment: 11 pages, Latex, epsf.sty included + 5 PostScript pictures