Nonperturbative QCD Phenomenology and Light Quark Physics

Recent progress in modeling QCD for hadron physics through truncated Dyson-Schwinger equations is reviewed. Special emphasis is put upon comparison of dressed quark propagators and the dressed quark-gluon vertex with lattice-QCD results.Comment: 6 pages, 7 figures. Invited talk at the QCD Down Under workshop at the CSSM/University of Adelaide, March 200

Covariant QCD Modeling of Light Meson Physics

We summarize recent progress in soft QCD modeling based on the set of Dyson--Schwinger equations truncated to ladder-rainbow level. This covariant approach to hadron physics accommodates quark confinement and implements the QCD one-loop renormalization group behavior. We compare the dressed quark propagator, pseudoscalar and vector meson masses as a function of quark mass, and the rho -> pi pi coupling to recent lattice-QCD data. The error in the Gell-Mann--Oakes--Renner relation with increasing quark mass is quantified by comparison to the exact pseudoscalar mass relation as evaluated within the ladder-rainbow Dyson-Schwinger model.Comment: Presented at the International School on Nuclear Physics, 24th course: Quarks in Nuclei, Erice, Sicily, September 2002; to be published in Prog. Part. Nucl. Phys.; 6 pages, 6 fig

Nucleon form factors and a nonpointlike diquark

Nucleon form factors are calculated on q^2 in [0,3] GeV^2 using an Ansatz for the nucleon's Fadde'ev amplitude motivated by quark-diquark solutions of the relativistic Fadde'ev equation. Only the scalar diquark is retained, and it and the quark are confined. A good description of the data requires a nonpointlike diquark correlation with an electromagnetic radius of 0.8 r_pi. The composite, nonpointlike nature of the diquark is crucial. It provides for diquark-breakup terms that are of greater importance than the diquark photon absorption contribution.Comment: 5 pages, REVTEX, epsfig, 3 figure

QCD modeling of hadron physics

We review recent developments in the understanding of meson properties as solutions of the Bethe-Salpeter equation in rainbow-ladder truncation. Included are recent results for the pseudoscalar and vector meson masses and leptonic decay constants, ranging from pions up to c\bar{c} bound states; extrapolation to b\bar{b} states is explored. We also present a new and improved calculation of F_\pi(Q^2) and an analysis of the \pi\gamma\gamma transition form factor for both \pi(140) and \pi(1330). Lattice-QCD results for propagators and the quark-gluon vertex are analyzed, and the effects of quark-gluon vertex dressing and the three-gluon coupling upon meson masses are considered.Comment: 17 pages, 19 postscript figures, contribution to the proceedings of LC05, Cairns, Australia, July 200

Diquarks: condensation without bound states

We employ a bispinor gap equation to study superfluidity at nonzero chemical potential: mu .neq. 0, in two- and three-colour QCD. The two-colour theory, QC2D, is an excellent exemplar: the order of truncation of the quark-quark scattering kernel: K, has no qualitative impact, which allows a straightforward elucidation of the effects of mu when the coupling is strong. In rainbow-ladder truncation, diquark bound states appear in the spectrum of the three-colour theory, a defect that is eliminated by an improvement of K. The corrected gap equation describes a superfluid phase that is semi-quantitatively similar to that obtained using the rainbow truncation. A model study suggests that the width of the superfluid gap and the transition point in QC2D provide reliable quantitative estimates of those quantities in QCD.Comment: 7 pages, 3 figures, REVTEX, epsfi

Random walk on disordered networks

Random walks are studied on disordered cellular networks in 2-and 3-dimensional spaces with arbitrary curvature. The coefficients of the evolution equation are calculated in term of the structural properties of the cellular system. The effects of disorder and space-curvature on the diffusion phenomena are investigated. In disordered systems the mean square displacement displays an enhancement at short time and a lowering at long ones, with respect to the ordered case. The asymptotic expression for the diffusion equation on hyperbolic cellular systems relates random walk on curved lattices to hyperbolic Brownian motion.Comment: 10 Pages, 3 Postscript figure

Half-life of the I Π = 4 - intruder state in 34 P: M 2 transition strengths approaching the island of inversion

The half-life of the 2305-keV, I?=4 (-) intruder state in 34P has been measured as t 1/2=2.0(1) ns using ?-ray coincidence, fast-timing techniques with the Bucharest HPGe and LaBr 3:Ce detector array. Excited states in 34P were populated using the 18O (18O,pn )34P fusion-evaporation reaction at a beam energy of 36 MeV. Reduced transition probabilities have been calculated for different values of the M2/E3 mixing ratio, ? E3/M2, and compared with shell-model estimates. For small values of the mixing ratio, the B(M2) value was found to be consistent with similar transitions associated with the occupation of neutron f 7/2 configurations in this mass region. � 2012 American Physical Society

Analytic properties of the Landau gauge gluon and quark propagators

We explore the analytic structure of the gluon and quark propagators of Landau gauge QCD from numerical solutions of the coupled system of renormalized Dyson--Schwinger equations and from fits to lattice data. We find sizable negative norm contributions in the transverse gluon propagator indicating the absence of the transverse gluon from the physical spectrum. A simple analytic structure for the gluon propagator is proposed. For the quark propagator we find evidence for a mass-like singularity on the real timelike momentum axis, with a mass of 350 to 500 MeV. Within the employed Green's functions approach we identify a crucial term in the quark-gluon vertex that leads to a positive definite Schwinger function for the quark propagator.Comment: 42 pages, 16 figures, revtex; version to be published in Phys Rev

Deep inelastic scattering off a N=4 SYM plasma at strong coupling

By using the AdS/CFT correspondence we study the deep inelastic scattering of an R-current off a N=4 supersymmetric Yang-Mills (SYM) plasma at finite temperature and strong coupling. Within the supergravity approximation valid when the number of colors is large, we compute the structure functions by solving Maxwell equations in the space-time geometry of the AdS_5 black three-brane. We find a rather sharp transition between a low energy regime where the scattering is weak and quasi-elastic, and a high-energy regime where the current is completely absorbed. The critical energy for this transition determines the plasma saturation momentum in terms of its temperature T and the Bjorken x variable: Q_s=T/x. These results suggest a partonic picture for the plasma where all the partons have transverse momenta below the saturation momentum and occupation numbers of order one.Comment: Version accepted for publication in JHEP: more references added; some technical points were displaced from Sect. 4 to the new Appendix