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Sep 20, 2013
09/13

by
Adriano Di Giacomo; Enrico Meggiolaro; Haralambos Panagopoulos

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We study by numerical simulations on a lattice the behaviour of the gauge--invariant two--point correlation functions of the gauge field strengths across the deconfinement phase transition.

Source: http://arxiv.org/abs/hep-lat/9603018v1

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35

Sep 19, 2013
09/13

by
Enrico Meggiolaro

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We shall discuss about some analytic properties of the high-energy parton-parton (and hadron-hadron) scattering amplitudes in gauge theories, when going from Minkowskian to Euclidean theory, and we shall see how they can be related to the still unsolved problem of the s-dependence of the total cross-section.

Source: http://arxiv.org/abs/hep-ph/0302182v2

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2.0

Jun 30, 2018
06/18

by
Matteo Giordano; Enrico Meggiolaro

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We show how to obtain rising hadronic total cross sections in QCD, in the framework of the nonperturbative approach to soft high-energy scattering based on Wilson-loop correlators. Total cross sections turn out to be of "Froissart"-type, i.e., the leading energy dependence is of the form $\sigma_{\rm tot} \sim B \log^2 s$, in agreement with experiments. The observed universality of the prefactor $B$ is obtained rather naturally in this framework. In this case, $B$ is entirely...

Topic: High Energy Physics - Phenomenology

Source: http://arxiv.org/abs/1409.7376

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2.0

Jun 29, 2018
06/18

by
Enrico Meggiolaro; Massimo D'Elia; Michele Mesiti; Francesco Negro

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We present the results of an exploratory study (by means of Monte Carlo simulations on the lattice) of the properties of the gauge-invariant two-point correlation functions of the gauge-field strengths for $N_f=2$ QCD at zero temperature and in the presence of a magnetic background field: the analysis provides evidence for the emergence of anisotropies in the nonperturbative part of the correlators and for an increase of the gluon condensate as a function of the external magnetic field.

Topics: High Energy Physics - Theory, High Energy Physics - Phenomenology, High Energy Physics - Lattice

Source: http://arxiv.org/abs/1611.02533

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4.0

Jun 28, 2018
06/18

by
Massimo D'Elia; Enrico Meggiolaro; Michele Mesiti; Francesco Negro

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We consider the properties of the gauge-invariant two-point correlation functions of the gauge-field strengths for QCD in the presence of a magnetic background field. We discuss the general structure of the correlators in this case and provide the results of an exploratory lattice study for $N_f = 2$ QCD discretized with unimproved staggered fermions. Our analysis provides evidence for the emergence of anisotropies in the non-perturbative part of the correlators and for an increase of the gluon...

Topics: High Energy Physics - Lattice, High Energy Physics - Phenomenology, High Energy Physics - Theory

Source: http://arxiv.org/abs/1510.07012

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2.0

Jun 30, 2018
06/18

by
Matteo Giordano; Enrico Meggiolaro

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We discuss how hadronic total cross sections at high energy depend on the details of QCD, namely on the number of colours $N_c$ and the quark masses. We find that while a "Froissart"-type behaviour $\sigma_{\rm tot}\sim B\log^2s$ is rather general, relying only on the presence of higher-spin stable particles in the spectrum, the value of $B$ depends quite strongly on the quark masses. Moreover, we argue that $B$ is of order ${\cal O}(N_c^0)$ at large $N_c$, and we discuss a bound for...

Topics: High Energy Physics - Phenomenology, Nuclear Theory

Source: http://arxiv.org/abs/1411.0553

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44

Sep 22, 2013
09/13

by
Enrico Meggiolaro

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We discuss the results obtained by fitting the lattice data of the gauge-invariant field strength correlators in QCD with some particular functions which are commonly used in the literature in some phenomenological approaches to high-energy hadron-hadron scattering. A comparison is done with the results obtained in the original fits to the lattice data.

Source: http://arxiv.org/abs/hep-ph/9807567v2

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37

Sep 19, 2013
09/13

by
Enrico Meggiolaro; Matteo Giordano

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We will discuss how the problem of high-energy hadron-hadron (dipole-dipole) scattering at low momentum transfer can be approached from the point of view of lattice QCD, by means of Monte Carlo numerical simulations.

Source: http://arxiv.org/abs/1010.0914v1

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4.0

Jun 30, 2018
06/18

by
Enrico Meggiolaro

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We analyze (using a chiral effective Lagrangian model) the scalar and pseudoscalar meson mass spectrum of QCD at finite temperature, above the chiral transition at $T_c$, looking, in particular, for signatures of a possible breaking of the $U(1)$ axial symmetry above $T_c$. A detailed comparison between the case with a number of light quark flavors $N_f\geq 3$ and the (remarkably different) case $N_f=2$ is performed.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Lattice

Source: http://arxiv.org/abs/1410.3734

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Sep 20, 2013
09/13

by
Adriano Di Giacomo; Enrico Meggiolaro; Haralambos Panagopoulos

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We determine by numerical simulations on a lattice the gauge--invariant two--point correlation function of the gauge field strengths in the QCD vacuum, down to a distance of 0.1 fm.

Source: http://arxiv.org/abs/hep-lat/9603017v1

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103

Jul 20, 2013
07/13

by
Enrico Meggiolaro

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By a direct resummation of perturbation theory in the limit of very high energy and small transferred momentum (the so-called ``eikonal'' limit), we derive expressions for the truncated-connected quark, antiquark and gluon propagators in an external gluon field, both for scalar and fermion gauge theories. These are the basic ingredients to derive ``soft'' high-energy parton-parton scattering amplitudes, using the LSZ reduction formulae and a functional integral approach.

Source: http://arxiv.org/abs/hep-ph/0107246v1

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Sep 23, 2013
09/13

by
Enrico Meggiolaro

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We study the asymptotic short-distance behaviour as well as the asymptotic large-distance behaviour of the gauge-invariant quark-antiquark nonlocal condensates in QCD. A comparison of some analytical results with the available lattice data is performed.

Source: http://arxiv.org/abs/hep-lat/9909068v2

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Sep 23, 2013
09/13

by
Adriano Di Giacomo; Enrico Meggiolaro; Haralambos Panagopoulos

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We study, by numerical simulations on a lattice, the behaviour of the gauge--invariant field strength correlators in QCD both at zero temperature, down to a distance of 0.1 fm, and at finite temperature, across the deconfinement phase transition.

Source: http://arxiv.org/abs/hep-lat/9608008v1

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48

Sep 20, 2013
09/13

by
Enrico Meggiolaro

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We discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. We expect that, above a certain critical temperature, also the U(1) axial symmetry will be restored. We will try to see if this transition has (or has not) anything to do with the usual chiral transition: various possible scenarios are discussed. In particular, we analyse a scenario in which the U(1) axial symmetry is still broken above the chiral transitions. We will show that this scenario can...

Source: http://arxiv.org/abs/hep-ph/0204312v1

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39

Jul 20, 2013
07/13

by
Enrico Meggiolaro

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It is known that the high-energy quark-quark scattering amplitude can be described by the expectation value of two lightlike Wilson lines, running along the classical trajectories of the two colliding particles. Generalizing the results of a previous paper, we give here the general proof that the expectation value of two infinite Wilson lines, forming a certain hyperbolic angle in Minkowski space-time, and the expectation value of two infinite Euclidean Wilson lines, forming a certain angle in...

Source: http://arxiv.org/abs/hep-th/9702186v2

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38

Sep 21, 2013
09/13

by
Massimo D'Elia; Adriano Di Giacomo; Enrico Meggiolaro

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We study, by numerical simulations on a lattice, the behaviour of the gauge-invariant two-point correlation functions of the gauge field strengths in the QCD vacuum with dynamical fermions.

Source: http://arxiv.org/abs/hep-lat/9705032v1

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Sep 19, 2013
09/13

by
Enrico Meggiolaro

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The high-energy quark-quark scattering amplitude is calculated first in the case of scalar QCD, using Fradkin's approach to derive the scalar quark propagator in an external gluon field and computing it in the eikonal approximation. The results are then extended to the case of ``real'' (i.e., fermion) QCD. The high-energy quark-quark scattering amplitude turns out to be described by the expectation value of two lightlike Wilson lines, running along the classical trajectories of the two...

Source: http://arxiv.org/abs/hep-th/9709045v1

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38

Sep 19, 2013
09/13

by
Enrico Meggiolaro

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We consider a scenario (supported by some lattice results) in which a U(1)-breaking condensate survives across the chiral transition in QCD. This scenario has important consequences for the pseudoscalar-meson sector, which can be studied using an effective Lagrangian model. In particular, generalizing the results obtained in two previous papers, where the effects on the radiative decays eta,eta' --> gamma gamma were studied, in this paper we study the effects of the U(1) chiral condensate on...

Source: http://arxiv.org/abs/1010.1140v3

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34

Sep 22, 2013
09/13

by
Enrico Meggiolaro

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In this paper we consider some analytic properties of the high--energy quark--quark scattering amplitude, which, as is well known, can be described by the expectation value of two lightlike Wilson lines, running along the classical trajectories of the two colliding particles. We shall prove that the expectation value of two infinite Wilson lines, forming a certain hyperbolic angle in Minkowski space--time, and the expectation value of two infinite Euclidean Wilson lines, forming a certain angle...

Source: http://arxiv.org/abs/hep-th/9602104v1

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38

Sep 19, 2013
09/13

by
Massimo D'Elia; Adriano Di Giacomo; Enrico Meggiolaro

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We determine, by numerical simulations on a lattice, the gauge-invariant two-point correlation functions of the gauge field strengths in the QCD vacuum with four flavours of dynamical staggered fermions.

Source: http://arxiv.org/abs/hep-lat/9709031v1

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Sep 18, 2013
09/13

by
Enrico Meggiolaro

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The high-energy parton-parton scattering amplitude can be described, in the c.m.s., by the expectation value of two infinite Wilson lines, running along the classical trajectories of the two colliding particles. The above description suffers from IR divergences (typical of 3 + 1 dimensional gauge theories), which can be regularized by considering finite Wilson lines, extending in proper time from -T to T (and eventually sending T to infinity). Generalizing the results of a previous paper, we...

Source: http://arxiv.org/abs/hep-ph/0110069v1

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36

Sep 18, 2013
09/13

by
Enrico Meggiolaro; Christof Wetterich

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A nonperturbative renormalization group equation describes how the momentum dependent four-quark vertex depends on an infrared cutoff. We find a quasilocal one-particle irreducible piece generated by (anomaly-free) multi-gluon exchange. It becomes important at a cutoff scale where scalar and pseudoscalar meson-bound states are expected to play a role. This interaction remains subleading as compared to the effective one-gluon exchange contribution. The local instanton induced four-quark...

Source: http://arxiv.org/abs/hep-ph/0012081v1

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40

Sep 19, 2013
09/13

by
Enrico Meggiolaro

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In this paper we calculate the high--energy quark--quark scattering amplitude, first in the case of scalar QCD, using Fradkin's approach to derive the scalar quark propagator in an external gluon field and computing it in the eikonal approximation. (This approach was also recently used by Fabbrichesi, Pettorino, Veneziano and Vilkovisky to study the four--dimensional Planckian--energy scattering in gravity.) We then extend the results to the case of ``real'' ({\it i.e.} fermion) QCD, thus...

Source: http://arxiv.org/abs/hep-th/9506043v2

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10.0

Jun 28, 2018
06/18

by
Matteo Giordano; Enrico Meggiolaro

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We determine the large-distance behaviour of the static dipole-dipole potential for a wide class of gauge theories on nonperturbative grounds, exploiting only general properties of the theory. In the case of QCD, we recover the known results in the regime of small dipole sizes, and discuss recent nonperturbative calculations. Moreover, we discuss the case of pure-gauge theories, and compare our prediction with the available lattice results.

Topics: High Energy Physics - Phenomenology, High Energy Physics - Lattice, High Energy Physics - Theory

Source: http://arxiv.org/abs/1507.07215

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40

Sep 20, 2013
09/13

by
Enrico Meggiolaro

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We show how the topological susceptibility in the Minkowskian theory of QCD is related to the corresponding quantity in the Euclidean theory, which is measured on the lattice. We discuss both the zero-temperature case (T = 0) and the finite-temperature case (T > 0). It is shown that the two quantities are equal when T = 0, while the relation between them is much less trivial when T > 0. The possible existence of ``Kogut-Susskind poles'' in the matrix elements of the topological charge...

Source: http://arxiv.org/abs/hep-th/9802114v1

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91

Jul 20, 2013
07/13

by
Matteo Giordano; Enrico Meggiolaro; Niccolò Moretti

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The nonperturbative approach to soft high-energy hadron-hadron scattering, based on the analytic continuation of Wilson-loop correlation functions from Euclidean to Minkowskian theory, allows to investigate the asymptotic energy dependence of hadron-hadron total cross sections in lattice QCD. In this paper we will show, using best fits of the lattice data with proper functional forms satisfying unitarity and other physical constraints, how indications emerge in favor of a universal asymptotic...

Source: http://arxiv.org/abs/1203.0961v2

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48

Sep 19, 2013
09/13

by
Enrico Meggiolaro

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This paper is organized in two parts. The first part (Sections 2-5) is dedicated to the theory at T=0 and contains a pedagogical review of some fundamental aspects related with the chiral symmetries of QCD, the U(1) problem and its solution proposed by 'tHooft, Witten and Veneziano. In the second part (Sections 6-14) we discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. One expects that, above a certain critical temperature, also the U(1) axial...

Source: http://arxiv.org/abs/hep-ph/0206236v1

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37

Sep 22, 2013
09/13

by
Enrico Meggiolaro

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We discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. In particular, supported by recent lattice results, we analyse a scenario in which a U(1)-breaking condensate survives across the chiral transition. This scenario can be consistently reproduced using an effective Lagrangian model. The effects of the U(1) chiral condensate on the slope of the topological susceptibility in the full theory with quarks are studied. Further information on the new...

Source: http://arxiv.org/abs/hep-ph/0309214v1

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100

Jul 20, 2013
07/13

by
Enrico Meggiolaro

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In this paper we consider soft high-energy parton-parton scattering processes in gauge theories, i.e., elastic scattering processes involving partons at very high squared energies in the center of mass and small squared transferred momentum. By a direct resummation of perturbation theory in the limit we are considering, we derive expressions for the truncated-connected quark (antiquark) propagator in an external gluon field, as well as for the residue at the pole of the full unrenormalized...

Source: http://arxiv.org/abs/hep-ph/0009261v1