Dynamical Mass Generation in Landau gauge QCD

We summarise results on the infrared behaviour of Landau gauge QCD from the Green's functions approach and lattice calculations. Approximate, nonperturbative solutions for the ghost, gluon and quark propagators as well as first results for the quark-gluon vertex from a coupled set of Dyson-Schwinger equations are compared to quenched and unquenched lattice results. Almost quantitative agreement is found for all three propagators. Similar effects of unquenching are found in both approaches. The dynamically generated quark masses are close to `phenomenological' values. First results for the quark-gluon vertex indicate a complex tensor structure of the non-perturbative quark-gluon interaction.Comment: 6 pages, 6 figures, Summary of a talk given at the international conference QCD DOWN UNDER, March 10 - 19, Adelaide, Australi

Rho and Sigma Mesons in Unitarized Thermal ChPT

We present our recent results for the rho and sigma mesons considered as resonances in pion-pion scattering in a thermal bath. We use chiral perturbation theory to fourth order in p for the low energy behaviour, then extend the analysis via the unitarization method of the Inverse Amplitude into the resonance region. The width of the rho broadens about twice the amount required by phase space considerations alone, its mass staying practically constant up to temperatures of order 150 MeV. The sigma meson behaves in accordance to chiral symmetry restoration expectations.Comment: Proc. Workshop Strong and Electroweak Matter 02, Heidelberg, German

The BES f_0(1810): a new glueball candidate

We analyze the f_0(1810) state recently observed by the BES collaboration via radiative J/\psi decay to a resonant \phi\omega spectrum and confront it with DM2 data and glueball theory. The DM2 group only measured \omega\omega decays and reported a pseudoscalar but no scalar resonance in this mass region. A rescattering mechanism from the open flavored KKbar decay channel is considered to explain why the resonance is only seen in the flavor asymmetric \omega\phi branch along with a discussion of positive C parity charmonia decays to strengthen the case for preferred open flavor glueball decays. We also calculate the total glueball decay width to be roughly 100 MeV, in agreement with the narrow, newly found f_0, and smaller than the expected estimate of 200-400 MeV. We conclude that this discovered scalar hadron is a solid glueball candidate and deserves further experimental investigation, especially in the K-Kbar channel. Finally we comment on other, but less likely, possible assignments for this state.Comment: 11 pages, 4 figures. Major substantive additions, including an ab-initio, QCD-based computation of the glueball inclusive decay width, evaluation of final state effects, and enhanced discussion of several alternative possibilities. Our conclusions are unchanged: the BES f_0(1810) is a promising glueball candidat

Quark mass dependence of the pion vector form factor

We examine the quark mass dependence of the pion vector form factor, particularly the curvature (mean quartic radius). We focus our study on the consequences of assuming that the coupling constant of the rho to pions, g(rho pi pi), is largely independent of the quark mass while the quark mass dependence of the rho mass is given by recent lattice data. By employing the Omnes representation we can provide a very clean estimate for a certain combination of the curvature and the square radius, whose quark mass dependence could be determined from lattice computations. This study provides an independent access to the quark mass dependence of the rho pi pi coupling and in this way a non-trivial check of the systematics of chiral extrapolations. We also provide an improved value for the curvature for physical values for the quark masses. namely (r(4)) = 0.73 +/- 0.09 fm(4) or equivalently c(v) = 4.00 +/- 0.50 GeV-4. (C) 2009 Elsevier B.V. All rights reserved

Tetraquark spectroscopy

A complete classification of tetraquark states in terms of the spin-flavor, color and spatial degrees of freedom was constructed. The permutational symmetry properties of both the spin-flavor and orbital parts of the quark-quark and antiquark-antiquark subsystems are discussed. This complete classification is general and model-independent, and is useful both for model-builders and experimentalists. The total wave functions are also explicitly constructed in the hypothesis of ideal mixing; this basis for tetraquark states will enable the eigenvalue problem to be solved for a definite dynamical model. This is also valid for diquark-antidiquark models, for which the basis is a subset of the one we have constructed. An evaluation of the tetraquark spectrum is obtained from the Iachello mass formula for normal mesons, here generalized to tetraquark systems. This mass formula is a generalizazion of the Gell-Mann Okubo mass formula, whose coefficients have been upgraded by means of the latest PDG data. The ground state tetraquark nonet was identified with $f_{0}(600)$, $\kappa(800)$, $f_{0}(980)$, $a_{0}(980)$. The mass splittings predicted by this mass formula are compared to the KLOE, Fermilab E791 and BES experimental data. The diquark-antidiquark limit was also studied.Comment: Invited talk at 11th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2007), Julich, Germany, 10-14 Sep 2007. In the Proceedings of 11th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2007), Julich, Germany, 10-14 Sep 2007, eConf C070910, 163 (2007

Effective potential between two gluons from the scalar glueball

Starting from the $0^{++}$ glueball mass and wave function computed from lattice QCD, we compute the local potential between two constituent gluons. Since the properties of constituent gluons are still a matter of research, we allow for them to be either massless, or massive with a mass around 0.7 GeV. Both pictures are actually used in the literature. When the gluons are massless, the corresponding local potential is shown to be compatible with a Cornell form, that is a linear confinement plus a short-range Coulomb part, with standard values for the flux tube energy density and for the strong coupling constant. When the gluons are massive, the confining potential is a saturating one, commonly used to simulate string-breaking effects. These results fill a gap between lattice QCD and phenomenological models: The picture of the scalar glueball as a bound state of two constituent gluons interacting via a phenomenological potential is shown to emerge from pure gauge lattice QCD computations. Moreover, we show that the allowed potential shape is constrained by the mass of the constituent gluons.Comment: 4 figures; Comments added, one typo corrected in v2. V3 accepted for publication in EPJA : major changes, content enlarged, inclusion of massive gluon