Soft QCD Modeling of Meson Electromagnetic Form Factors

We summarize recent progress in soft QCD modeling based on the set of Dyson--Schwinger equations truncated to ladder-rainbow level. We pay particular attention to electromagnetic elastic and transition form factors of the pion. This covariant approach accommodates quark confinement and implements the QCD one-loop renormalization group behavior. The dressed quark propagators are compared to the most recent lattice-QCD data.Comment: For Proceedings of the Workshop on Lepton Scattering, Hadrons, and QCD, Adelaide, Australia, March-April 2001; LaTeX, 9 pages, 5 eps figures, WS style file included; to be publishe

Parton Decomposition of Nucleon Spin and Momentum: Gluons from Dressed Quarks

The lowest two Mellin moments of hadronic Generalized Parton Distributions are explored within a model that allows investigation of the inter-related quark and gluon contributions. For light quarks their dynamical connection is strong due to quark dressing. Our principal focus is the angular momentum $J$ of the nucleon. This work employs and extends dynamical insights obtained from our recent model results for quark and gluon momentum fractions $\langle x \rangle_{\rm q/g}$ in both pion and nucleon. The employed model is based on the Rainbow-Ladder truncation of the Dyson-Schwinger equations of QCD. The special case of a 1-loop treatment of a single hadronic quark is used to motivate several insights and obtain initial estimates such as the Wilson line correction to the established Landau gauge model ($-7\%$ for both $J_{\rm q}$ and $\langle x \rangle_{\rm q}$), and the "binding gluon" contribution to $\langle x \rangle_{\rm g}$ ($\leq 10\%$). We obtain the proton $J$ within 1\% after inclusion of the pion cloud mechanism to produce the sea. The gluon second Mellin moments of the proton reflect similar dynamics; ($\langle x \rangle_{\rm g}$, $J_{\rm g}$) are (26\% , 24\%) at model scale, and (40\%, 38\%) at $2$ GeV. We also find that $J_{\rm tot}$ is shared almost equally between the total orbital and total intrinsic spin contributions.Comment: 8 pages, 5 figure

Elektromagnetski faktori oblika mezonskih prijelaza

Within the Dyson-Schwinger-equation approach to modeling QCD for meson physics, we present new results for gρππ and the coupling constants and form factors for the transitions γ ∗πρ and γ ∗π 0γ. We discuss the role of the sub-dominant covariants of the π Bethe-Salpeter amplitude and investigate the asymptotic behavior of the γ ∗π 0γ form factor.Polazeći od Dyson-Schwingerove jednadžbe za modeliranje QCD mezonske fizike, predstavljaju se novi ishodi za gρππ, stalnice vezanja i faktore oblika prijelaza γ ∗πρ i γ ∗π 0γ. Raspravlja se uloga pod-dominantne kovarijante π Bethe-Salpeterove amplitude i istražuje asimptotsko ponašanje faktora oblika γ ∗π 0γ

Elektromagnetski faktori oblika mezonskih prijelaza

Within the Dyson-Schwinger-equation approach to modeling QCD for meson physics, we present new results for gρππ and the coupling constants and form factors for the transitions γ ∗πρ and γ ∗π 0γ. We discuss the role of the sub-dominant covariants of the π Bethe-Salpeter amplitude and investigate the asymptotic behavior of the γ ∗π 0γ form factor.Polazeći od Dyson-Schwingerove jednadžbe za modeliranje QCD mezonske fizike, predstavljaju se novi ishodi za gρππ, stalnice vezanja i faktore oblika prijelaza γ ∗πρ i γ ∗π 0γ. Raspravlja se uloga pod-dominantne kovarijante π Bethe-Salpeterove amplitude i istražuje asimptotsko ponašanje faktora oblika γ ∗π 0γ

QCD dynamics in mesons at soft and hard scales

Using a ladder-rainbow kernel previously established for the soft scale of light quark hadrons, we explore, within a Dyson-Schwinger approach, phenomena that mix soft and hard scales of QCD. The difference between vector and axial vector current correlators is examined to estimate the four quark chiral condensate and the leading distance scale for the onset of non-perturbative phenomena in QCD. The valence quark distributions, in the pion and kaon, defined in deep inelastic scattering, and measured in the Drell Yan process, are investigated with the same ladder-rainbow truncation of the Dyson-Schwinger and Bethe-Salpeter equations.Comment: 6 pages, 1 double panel figure, invited talk presented at the Workshop on Achievements and New Directions in Subatomic Physics, Adelaide, Australia, February 2010, to be published by AIP Conf. Pro

Selected highlights from the study of mesons

We provide a brief review of recent progress in the study of mesons using QCD's Dyson-Schwinger equations. Along the way we touch on aspects of confinement and dynamical chiral symmetry breaking but in the main focus upon: exact results for pseudoscalar mesons, including aspects of the eta-eta' problem; a realisation that the so-called vacuum condensates are actually an intrinsic, localised property of hadrons; an essentially nonperturbative procedure for constructing a symmetry-preserving Bethe-Salpeter kernel, which has enabled a demonstration that dressed-quarks possess momentum-dependent anomalous chromo- and electromagnetic moments that are large at infrared momenta, and resolution of a longstanding problem in understanding the mass-splitting between rho- and a1-mesons such that they are now readily seen to be parity partners in the meson spectrum; features of electromagnetic form factors connected with charged and neutral pions; and computation and explanation of valence-quark distribution functions in pseudoscalar mesons. We argue that in solving QCD, a constructive feedback between theory and extant and forthcoming experiments will enable constraints to be placed on the infrared behaviour of QCD's beta-function, the nonperturbative quantity at the core of hadron physics.Comment: 28 pages, 15 figures, 2 tables. Version to appear in the Chinese Journal of Physic