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Dalibor Kekez

Publications and source records attributed to Dalibor Kekez.

At least 19 recordsLinked to original sources
arXiv2023

Pion observables calculated in Minkowski and Euclidean spaces with Ansätze for quark propagators

We study two quark--propagator meromorphic Ansätze that admit clear connection between calculations in Euclidean space and Minkowski spacetime. The connection is established through a modified Wick rotation in momentum space, where the integration contour along the imaginary axis is adequately deformed. The Ansätze were previously proposed in the literature and fitted to Euclidean lattice QCD data. The generalized impulse approximation is used to calculate the pion transition form factor and electromagnetic form factor, correcting an earlier result. The pion decay constant and distribution amplitude are also calculated. The latter is used to deduce the asymptotic behavior of the form factors. Such an asymptotic behavior is compared with those obtained directly from the generalized impulse approximation and the causes of differences are pointed out.

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arXiv2020

Aspects of model dependence of eta'-eta complex treated by going beyond the isospin limit

Exploring the extent of model dependence, we study effects of certain Ansaetze for the T-dependence of the correction term in the QCD topological susceptibility. The one producing unwanted effects on results at T > 0 in the eta'-eta complex in the usual limit of isospin symmetry, is largely cured from its peculiar behavior and brought into agreement with the other, when breaking of isospin symmetry is allowed and the realistic quark mass ratio m_u/m_d = 0.50 is adopted.

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arXiv2020

A Dyson-Schwinger model beyond isospin limit prepared for investigating U_A(1)-breaking temperature dependence

Motivated by our earlier findings of sensitive quark-flavor dependence of QCD topological susceptibility on products of current quark masses and corresponding condensates, we allow the breaking of isospin symmetry. For the purpose of future investigations of U_A(1) symmetry breaking and restoration at T > 0, we perform (at T = 0) refitting of the quark-mass parameters of a phenomenologically successful effective model of low-energy QCD. It belongs to the class of separable-interaction models within the Dyson-Schwinger approach to the quark-antiquark substructure of mesons.

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arXiv2019

T-dependence of the axion mass when the U_A(1) and chiral symmetry breaking are tied

Modulo the scale of spontaneous breaking of Peccei-Quinn symmetry, the axion mass $m_a(T)$ is given by the QCD topological susceptibility $χ(T)$ at all temperatures $T$. From an approach tying the $U_A(1)$ and chiral symmetry breaking and getting good $T$-dependence of $η$ and $η$ mesons, we get $χ(T)$ for an effective Dyson-Schwinger model of nonperturbative QCD. Comparison with lattice results for $χ(T)$, and thus also for $m_a(T)$, shows good agreement for temperatures ranging from zero up to the double of the chiral restoration temperature $T_c$.

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arXiv2019

Temperature Dependence of the Axion Mass in a Scenario Where the Restoration of Chiral Symmetry Drives the Restoration of the $U_A(1)$ Symmetry

The temperature ($T$) dependence of the axion mass is predicted for $T$'s up to $\sim 2.3 \times$ the chiral restoration temperature of QCD. The axion is related to the $U_A(1)$ anomaly. The squared axion mass $m_a(T)^2$ is, modulo the presently undetermined scale of spontaneous breaking of Peccei-Quinn symmetry $f_a$ (squared), equal to QCD topological susceptibility $χ(T)$ for all $T$. We obtain $χ(T)$ by using quark condensates calculated in two effective Dyson-Schwinger models of nonperturbative QCD. They exhibit the correct chiral behavior, including the dynamical breaking of chiral symmetry and its restoration at high $T$. This is reflected in the $U_A(1)$ symmetry breaking and restoration through $χ(T)$. In our previous studies, such $χ(T)$ yields the $T$-dependence of the $U_A(1)$-anomaly-influenced masses of $η'$ and $η$ mesons consistent with experiment. This in turn supports our prediction for the $T$-dependence of the axion mass. Another support is a rather good agreement with the pertinent lattice results. This agreement is not spoiled by our varying $u$ and $d$ quark mass parameters out of the isospin limit.

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arXiv2018

$η'$ and $η$ mesons at high T when the U_A(1) and chiral symmetry breaking are tied

The approach to the eta'-eta complex employing chirally well-behaved quark-antiquark bound states and incorporating the non-Abelian axial anomaly of QCD through the generalization of the Witten-Veneziano relation, is extended to finite temperatures. Employing the chiral condensate has led to a sharp chiral and U_A(1) symmetry restoration, but with the condensates of quarks with realistic explicit chiral symmetry breaking, which exhibit a smooth, crossover chiral symmetry restoration in qualitative agreement with lattice QCD results, we get a crossover U_A(1) transition, with smooth and gradual melting of anomalous mass contributions. This way we obtain a substantial drop of the eta' mass around the chiral transition temperature, but no eta mass drop. This is consistent with the present empirical evidence.

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arXiv2014

Forbidden and invisible Z boson decays in covariant theta-exact noncommutative standard model

The triple neutral gauge boson and direct U(1)_Y-neutrino interactions, being absent in ordinary field theory, can arise quite naturally in noncommutative gauge field theories. Using non-perturbative methods and a Seiberg-Witten map based covariant approach to noncommutative gauge theory, we have found theta-exact expressions for both interactions, thereby eliminating previous restrictions to low-energy phenomena. In particular we obtain for the first time the covariant, theta-exact, triple neutral gauge boson interactions within the noncommutative Standard Model gauge sector including an additional gauge-field deformation freedom. Finally we discuss implications for Z->2gamma and Z->neutrino-pair decays, and show that our results behave quite reasonably throughout all interaction energy scales.

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arXiv2005

A Bethe-Salpeter study with the -enhanced effective QCD coupling

Dyson-Schwinger equations provide a prominent approach to physics of strong interactions. To reproduce the hadronic phenomenology well, the Dyson-Schwinger approach in the rainbow-ladder approximation must employ an effective interaction between quarks which is fairly strong at intermediate (Q^2 approx. 0.5 GeV^2) spacelike transferred momenta. We have recently proposed that such an interaction may originate from the dimension 2 gluon condensate which has recently attracted much attention, and showed that the resulting effective running coupling leads to the sufficiently strong dynamical chiral symmetry breaking and successful phenomenology at least in the light sector of pseudoscalar mesons. In the present paper, we give a more detailed investigation of the parameter dependence of these results.

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arXiv2004

Pseudoscalar qqbar mesons and effective QCD coupling enhanced by condensate

Recent developments provided evidence that the dimension 2 gluon condensate is important for the nonperturbative regime of Yang-Mills theories (quantized in the Landau gauge). We show that it may be relevant for the Dyson-Schwinger approach to QCD. In order that this approach leads to a successful hadronic phenomenology, an enhancement of the effective quark-gluon interaction seems to be needed at intermediate (p^2 \sim 0.5 GeV^2) momenta. It is shown that the gluon condensate provides such an enhancement. It is also shown that the resulting effective strong running coupling leads to the sufficiently strong dynamical chiral symmetry breaking and successful phenomenology at least in the light sector of pseudoscalar mesons.

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arXiv2002

Phenomenological Schwinger-Dyson approach to eta-eta' mass matrix

It is reviewed pedagogically how a very successful description of the eta-eta' mass matrix can be achieved in the consistently coupled Schwinger-Dyson and Bethe-Salpeter approach in spite of the limitations of the ladder approximation. This description is in agreement with both phenomenology and lattice results.

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arXiv2002

Schwinger-Dyson approach to light pseudoscalars

We review briefly some of the successes of Schwinger-Dyson (SD) approach to the physics of quarks and hadrons, primarily light pseudoscalar mesons including eta and eta'. The main purpose of this paper is to point out that SD results on eta and eta' mesons can be formulated and given also in the two-mixing-angle scheme.

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arXiv2001

Dynamical SU(3) linear sigma model and the mixing of eta'-eta and sigma-f_0 mesons

The SU(3) linear sigma model is dynamically generated in loop-order using the nonstrange-strange basis. Only self-consistent logarithmic divergent graphs are needed, with quadratic divergent graphs replaced by SU(3) mass-shell equal splitting laws. The latter lead to an eta'-eta mixing angle of 41.84 deg which is consistent with phenomenology. Finally this above SU(3) linear sigma model in turn predicts strong decay rates which are all compatible with data.

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arXiv2000

On the eta-eta' complex in the SD-BS approach

The bound-state Schwinger-Dyson and Bethe-Salpeter (SD-BS) approach is chirally well-behaved and provides a reliable treatment of the eta-eta' complex although a ladder approximation is employed. Allowing for the effects of the SU(3) flavor symmetry breaking in the quark-antiquark annihilation, leads to the improved eta-eta' mass matrix.

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arXiv1999

Schwinger-Dyson approach and generalized impulse approximation for the pi0 gamma^* gamma transition

We review the pion-photon transition form factor calculated in the Schwinger-Dyson approach and an impulse approximation. We present results for it far above the scales presently accessible for measurement, up to 36 GeV^2, and demonstrate agreement with the analytically inferred asymptotic behavior, for which we also provide a new derivation. We discuss how measurements at Jefferson Lab can provide information on how quarks are dynamically dressed.

hep-ph↗
arXiv1999

gamma^* gamma -> pi^0 transition and asymptotics of gamma^* gamma and gamma^* gamma^* transitions of other unflavored pseudoscalar mesons

For the spacelike momenta k of the virtual photon gamma^*, the pi^0(p) gamma^*(k) gamma(k') transition form factor is considered in the coupled Schwinger-Dyson and Bethe-Salpeter approach in conjunction with the generalized impulse approximation using the dressed quark-photon-quark vertices of the Ball-Chiu and Curtis-Pennington type. These form factors are compared with the ones predicted by the vector meson dominance, operator product expansion, QCD sum rules, and the perturbative QCD for the large spacelike transferred momenta k. The most important qualitative feature of the asymptotic behavior, namely the 1/k^2 dependence, is in our approach obtained in the model-independent way. Again model-independently, our approach reproduces also the Adler-Bell-Jackiw anomaly result for the limit of both photons being real. For the case of one highly virtual photon, we find in the closed form the asymptotic expression which can be easily generalized both to the case of other unflavored pseudoscalar mesons P^0 = pi^0, eta_8, eta_0, eta_c, eta_b, and to the case of arbitrary virtuality of the other photon. Implications thereof for certain important theoretical and experimental applications are pointed out.

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