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J. Papavassiliou

Publications and source records attributed to J. Papavassiliou.

At least 19 recordsLinked to original sources
arXiv2026

Real poles with opposite-sign residues in the non-perturbative quark propagator

We investigate the analytic structure of the quark propagator in the Landau gauge by dynamically coupling the standard gap equation to the non-perturbative quark-gluon vertex. Employing the full vertex basis, we demonstrate that for sub-GeV time-like momenta, the proper inclusion of the underlying dynamics leads to a pair of real poles with opposite-sign residues. In particular, in stark contradistinction to the results obtained in widely used approximations, we see no sign of complex conjugate poles. This distinctive analytic structure evades conceptual shortcomings frequently associated with complex conjugate poles while remaining fully compatible with the aspects of color confinement related to positivity violation. Crucially, this novel behavior is governed by a dominant triplet of vertex form factors: the tree-level component, the anomalous chromomagnetic moment, and a component we label as "spin-momentum curvature". By gradually tuning the individual strengths of these components, we demonstrate that while they contribute in distinct ways to the quark propagator, their joint action is vital for stabilizing the system. Together, they place the low-lying poles onto the real axis while producing a robust constituent quark mass of $350$ MeV.

hep-ph↗
arXiv2026

Quark-gluon vertex in the complex plane

In the present work we explore for the first time the general structure and properties of the nonperturbative quark-gluon vertex in the complex plane. Specifically, we focus on the transversely-projected quark-gluon vertex that emerges from a recently developed symmetry-preserving approach for the study of meson properties beyond the rainbow-ladder approximation. The analysis focuses on the so-called "soft-gluon" limit, which reduces the momentum-dependence of the corresponding vertex form factors to a single momentum variable. The complexification of this variable inside the defining integrals furnishes unambiguously all eight vertex form factors within a concrete domain of the complex variable, delimited by a characteristic parabola. The extent of this reliable domain is determined by the appearance of the first singularity in the integrands of the vertex integrals, where the standard Wick rotation must be duly supplemented by additional crucial contributions. This primary analytic region may be extended considerably by resorting to standard extrapolation methods, which remain valid up until the appearance of complex structures associated with the onset of physical processes. The generalization of the method to arbitrary gluon momenta, and its relevance for the determination of the quark propagator in the complex plane, are briefly discussed.

hep-ph↗
arXiv2026

Light mesons in the symmetric-vertex approximation

We compute the spectrum of light mesons, composed by up, down, and strange quarks, using a symmetry-preserving approximation that permits the inclusion of fully-dressed quark-gluon vertices in the key dynamical equations. This method is characterized by the use of the standard symmetric kinematic configuration as a seed in the corresponding Schwinger-Dyson equation, yielding finally the full kinematic dependence of all eight form factors composing the transversely-projected quark-gluon vertex. The extension of this approach to the case of distinct nonvanishing current quark masses is discussed, and the compatibility with the fundamental Ward-Takahashi identities demonstrated. The corresponding Bethe-Salpeter kernel is composed by three different diagrammatic structures, which may be deduced from the attendant quark gap equation by applying the standard "cutting" rules. The masses of the light mesons are computed by first determining the eigenvalue of the Bethe-Salpeter equation as a function of Euclidean momenta, and then using the Schlessinger extrapolation method to determine the Minkowski momentum for which this eigenvalue becomes unity. The resulting meson masses are in good agreement with experimental values, and substantially improve upon predictions from the rainbow-ladder approximation.

hep-ph↗
arXiv2026

Pions reloaded

We present a novel version of the pion Bethe-Salpeter equation in the chiral limit, solved using as ingredients state-of-the-art QCD correlation functions. The constraints imposed by the axial Ward-Takahashi identities are exactly fulfilled, both formally and numerically.

hep-ph↗
arXiv2026

Baryonic form factors of light pseudoscalar mesons

Employing the Bethe-Salpeter formalism, we present a computation of the space-like baryonic form factor for the pion and kaon. In the exact isospin-symmetric limit this observable is forbidden by $G$-parity, so that any nonzero signal constitutes a direct probe of the quark mass difference $m_d - m_u$. The form factors are evaluated in the impulse approximation using fully dressed quark propagators, meson Bethe-Salpeter amplitudes, and a dressed baryon-current vertex constrained by the vector Ward-Takahashi identity. The baryonic radius computed with this method for the pion is given by $\langle r_{\! B}^2\rangle_{π^+}^{1/2} = 0.043(2)$ fm, and is consistent with the available dispersive benchmarks. Our predictions for the kaons, namely $\langle r_{\!B}^2\rangle_{K^+}^{1/2} = 0.265(7)$ fm and $\langle r_{\!B}^2\rangle_{K^0}^{1/2} = 0.262(7)$ fm, indicate a larger spatial extent than in the pion case; these results have no dispersive counterparts, and are compatible with chiral QCD models.

hep-ph↗
arXiv2025

Electromagnetic properties of heavy-light mesons

Within the Bethe-Salpeter framework, we present a computation of space-like electromagnetic form factors for pseudoscalar mesons, including light and heavy-light systems. Our approach employs a flavour-dependent variation of the standard Taylor effective charge, which contains key contributions from the quark-gluon vertices. This effective interaction is a common ingredient of all relevant dynamical equations, and accommodates the crucial mass differences between the various quark flavours. Particular attention is paid to the nonperturbative determination of the quark-photon vertex. The computed electromagnetic form factors for the pion and the kaon mesons show excellent agreement with experimental determinations. In addition, the predictions for the charge radii of heavy-light systems are in overall good agreement with lattice QCD.

hep-ph↗
arXiv2025

Gluon mass scale through the Schwinger mechanism

It has long been argued that the action of the Schwinger mechanism in the gauge sector of Quantum Chromodynamics leads to the generation of a gluon mass scale. Within this scenario, the analytic structure of the fundamental vertices is modified by the creation of scalar colored excitations with vanishing mass. In the limit of zero momentum transfer, these terms act as massless poles, providing the required conditions for the infrared stabilization of the gluon propagator, and producing a characteristic displacement to the associated Ward identities. In this article we offer an extensive overview of the salient notions and techniques underlying this dynamical picture. We place particular emphasis on recent developments related to the exact renormalization of the mass, the nonlinear nature of the pole equation, and the key role played by the Fredholm alternative theorem.

hep-ph↗
arXiv2024

Infrared properties of the quark-gluon vertex in general kinematics

In the present work we determine the eight form factors of the transversely-projected quark-gluon vertex in general kinematics, in the context of Landau-gauge QCD with two degenerate light dynamical quarks. The study is based on the set of Schwinger-Dyson equations that govern the vertex form factors, derived within the formalism of the three-particle-irreducible (3PI) effective action. The analysis is performed by employing lattice data for the main ingredients, such as gluon and quark propagators, and three-gluon vertex. The numerical treatment is simplified by decoupling the system of integral equations: the classical form factor is determined from a single non-linear equation involving only itself, while the remaining ones are subsequently computed through simple integrations. The form factors are obtained for arbitrary values of space-like momenta, and their angular dependence is examined in detail. A clear hierarchy is established at the level of the corresponding dimensionless effective couplings, in agreement with results of earlier studies. Furthermore, the classical form factor is found to be in excellent agreement with recent unquenched lattice data in the soft-gluon configuration, while the two non-classical dressings depart substantially from the lattice results. Finally, the accurate implementation of multiplicative renormalizability is confirmed, and the transition from Minkoswski to Euclidean space is elucidated.

hep-ph↗
arXiv2024

Nonperturbative four-gluon vertex in soft kinematics

We present a nonperturbative study of the form factor associated with the projection of the full four-gluon vertex on its classical tensor, for a set of kinematics with one vanishing and three arbitrary external momenta. The treatment is based on the Schwinger-Dyson equation governing this vertex, and a large-volume lattice simulation, involving ten thousand gauge field configurations. The key hypothesis employed in both approaches is the ``planar degeneracy'', which classifies diverse configurations by means of a single variable, thus enabling their meaningful ``averaging''. The results of both approaches show notable agreement, revealing a considerable suppression of the averaged form factor in the infrared. The deviations from the exact planar degeneracy are discussed in detail, and a supplementary variable is used to achieve a more accurate description. The effective charge defined through this special form factor is computed within both approaches, and the results obtained are in excellent agreement.

hep-ph↗
arXiv2024

MoEDAL search in the CMS beam pipe for magnetic monopoles produced via the Schwinger effect

We report on a search for magnetic monopoles (MMs) produced in ultraperipheral Pb--Pb collisions during Run-1 of the LHC. The beam pipe surrounding the interaction region of the CMS experiment was exposed to 184.07 \textmu b$^{-1}$ of Pb--Pb collisions at 2.76 TeV center-of-mass energy per collision in December 2011, before being removed in 2013. It was scanned by the MoEDAL experiment using a SQUID magnetometer to search for trapped MMs. No MM signal was observed. The two distinctive features of this search are the use of a trapping volume very close to the collision point and ultra-high magnetic fields generated during the heavy-ion run that could produce MMs via the Schwinger effect. These two advantages allowed setting the first reliable, world-leading mass limits on MMs with high magnetic charge. In particular, the established limits are the strongest available in the range between 2 and 45 Dirac units, excluding MMs with masses of up to 80 GeV at 95\% confidence level.

nucl-ex↗
arXiv2024

Nonlinear Schwinger mechanism in QCD, and Fredholm alternatives theorem

We present a novel implementation of the Schwinger mechanism in QCD, which fixes uniquely the scale of the effective gluon mass and streamlines considerably the procedure of multiplicative renormalization. The key advantage of this method stems from the nonlinear nature of the dynamical equation that generates massless poles in the longitudinal sector of the three-gluon vertex. An exceptional feature of this approach is an extensive cancellation involving the components of the integral expression that determines the gluon mass; it is triggered once the Schwinger-Dyson equation of the pole-free part of the three-gluon vertex has been appropriately exploited. It turns out that this cancellation is driven by the so-called Fredholm alternatives theorem, which operates among the set of integral equations describing this system. Quite remarkably, in the linearized approximation this theorem enforces the exact masslessness of the gluon. Instead, the nonlinearity induced by the full treatment of the relevant kernel evades this theorem, allowing for the emergence of a nonvanishing mass. The numerical results obtained from the resulting equations are compatible with the lattice findings, and may be further refined through the inclusion of the remaining fundamental vertices of the theory.

hep-ph↗
arXiv2024

Lattice determination of the Batalin-Vilkovisky function and the strong running interaction

The Batalin-Vilkovisky function is a central component in the modern formulation of the background field method and the physical applications derived from it. In the present work we report on novel lattice results for this particular quantity, obtained by capitalizing on its equality with the Kugo-Ojima function in the Landau gauge. The results of the lattice simulation are in very good agreement with the predictions derived from a continuum analysis based on the corresponding Schwinger-Dyson equations. In addition, we show that an important relation connecting this function with the ghost propagator is fulfilled rather accurately. With the aid of these results, we carry out the first completely lattice-based determination of the process-independent strong running interaction, employed in a variety of phenomenological studies.

hep-lat↗
arXiv2024

Four-gluon vertex in collinear kinematics

To date, the four-gluon vertex is the least explored component of the QCD Lagrangian, mainly due to the vast proliferation of Lorentz and color structures required for its description. In this work we present a nonperturbative study of this vertex, based on the one-loop dressed Schwinger-Dyson equation obtained from the 4PI effective action. A vast simplification is brought about by resorting to ``collinear'' kinematics, where all momenta are parallel to each other, and by appealing to the charge conjugation symmetry in order to eliminate certain color structures. Out of the fifteen form factors that comprise the transversely-projected version of this vertex, two are singled out and studied in detail; the one associated with the classical tensorial structure is moderately suppressed in the infrared regime, while the other diverges logarithmically at the origin. Quite interestingly, both form factors display the property known as ``planar degeneracy'' at a rather high level of accuracy. With these results we construct an effective charge that quantifies the strength of the four-gluon interaction, and compare it with other vertex-derived charges from the gauge sector of QCD.

hep-ph↗
arXiv2023

Schwinger displacement of the quark-gluon vertex

The action of the Schwinger mechanism in pure Yang-Mills theories endows gluons with an effective mass, and, at the same time, induces a measurable displacement to the Ward identity satisfied by the three-gluon vertex. In the present work we turn to Quantum Chromodynamics with two light quark flavors, and explore the appearance of this characteristic displacement at the level of the quark-gluon vertex. When the Schwinger mechanism is activated, this vertex acquires massless poles, whose momentum-dependent residues are determined by a set of coupled integral equations. The main effect of these residues is to displace the Ward identity obeyed by the pole-free part of the vertex, causing modifications to its form factors, and especially the one associated with the tree-level tensor. The comparison between the available lattice data for this form factor and the Ward identity prediction reveals a marked deviation, which is completely compatible with the theoretical expectation for the attendant residue. This analysis corroborates further the self-consistency of this mass-generating scenario in the general context of real-world strong interactions.

hep-ph↗
arXiv2023

Schwinger poles of the three-gluon vertex: symmetry and dynamics

The implementation of the Schwinger mechanism endows gluons with a nonperturbative mass through the formation of special massless poles in the fundamental QCD vertices; due to their longitudinal character, these poles do not cause divergences in on-shell amplitudes, but induce detectable effects in the Green's functions of the theory. Particularly important in this theoretical setup is the three-gluon vertex, whose pole content extends beyond the minimal structure required for the generation of a gluon mass. In the present work we analyze these additional pole patterns by means of two distinct, but ultimately equivalent, methods: the Slavnov-Taylor identity satisfied by the three-gluon vertex, and the nonlinear Schwinger-Dyson equation that governs the dynamical evolution of this vertex. Our analysis reveals that the Slavnov-Taylor identity imposes strict model-independent constraints on the associated residues, preventing them from vanishing. Approximate versions of these constraints are subsequently recovered from the Schwinger-Dyson equation, once the elements responsible for the activation of the Schwinger mechanism have been duly incorporated. The excellent coincidence between the two approaches exposes a profound connection between symmetry and dynamics, and serves as a nontrivial self-consistency test of this particular mass generating scenario.

hep-ph↗
arXiv2023

Planar degeneracy of the three-gluon vertex

We present a detailed exploration of certain outstanding features of the transversely-projected three-gluon vertex, using the corresponding Schwinger-Dyson equation in conjunction with key results obtained from quenched lattice simulations. The main goal of this study is the scrutiny of the approximate property denominated ``planar degeneracy'', unveiled when the Bose symmetry of the vertex is properly exploited. The planar degeneracy leads to a particularly simple parametrization of the vertex, reducing its kinematic dependence to essentially a single variable. Our analysis, carried out in the absence of dynamical quarks, reveals that the planar degeneracy is particularly accurate for the description of the form factor associated with the classical tensor, for a wide array of arbitrary kinematic configurations. Instead, the remaining three form factors display considerable violations of this property. In addition, and in close connection with the previous point, we demonstrate the numerical dominance of the classical form factor over all others, except in the vicinity of the soft-gluon kinematics. The final upshot of these considerations is the emergence of a very compact description for the three-gluon vertex in general kinematics, which may simplify significantly nonperturbative applications involving this vertex.

hep-ph↗
arXiv2023

Gauge Sector Dynamics in QCD

The dynamics of the gauge sector of QCD give rise to nonperturbative phenomena that are crucial for the internal consistency of the theory; most notably, they account for the generation of a gluon mass through the action of the Schwinger mechanism, the taming of the Landau pole and the ensuing stabilization of the gauge coupling, and the infrared suppression of the three-gluon vertex. In the present work, we review some key advances in the ongoing investigation of this sector within the framework of the continuum Schwinger function methods, supplemented by results obtained from lattice simulations.

hep-ph↗