Effective masses of diquarks

We study meson and diquark bound states using the rainbow-ladder truncation of QCD's Dyson-Schwinger equations. The infrared strength of the rainbow-ladder kernel is described by two parameters. The ultraviolet behavior is fixed by the one-loop renormalization group behavior of QCD, which ensures the correct asymptotic behavior of the Bethe-Salpeter amplitudes and brings important qualitative benefits. The diquark with the lowest mass is the scalar, followed by the axialvector and pseudoscalar diquark. This ordering can be anticipated from the meson sector.Comment: 14 pages, 4 figures, to appear in Few-Body System

Strong Decays of Light Vector Mesons

The vector meson strong decays rho-->pi pi, phi-->KK, and K^star-->pi K are studied within a covariant approach based on the ladder-rainbow truncation of the QCD Dyson--Schwinger equation for the quark propagator and the Bethe--Salpeter equation for the mesons. The model preserves the one-loop behavior of QCD in the ultraviolet, has two infrared parameters, and implements quark confinement and dynamical chiral symmetry breaking. The 3-point decay amplitudes are described in impulse approximation. The Bethe--Salpeter study motivates a method for estimating the masses for heavier mesons within this model without continuing the propagators into the complex plane. We test the accuracy via the rho, phi and K^{star} masses and then produce estimates of the model results for the a_1 and b_1 masses as well as the mass of the proposed exotic vector pi_1(1400).Comment: Submitted for publication; 10x2-column pages, REVTEX 4, 3 .eps files making 3fig

Hadron Physics and the Dyson--Schwinger Equations of QCD

We use the Bethe-Salpeter equation in rainbow-ladder truncation to calculate the ground state mesons from the chiral limit to bottomonium, with an effective interaction that was previously fitted to the chiral condensate and pion decay constant. Our results are in reasonable agreement with the data, as are the vector and pseudoscalar decay constants. The meson mass differences tend to become constant in the heavy-quark limit. We also present calculations for the pion and rho electromagnetic form factors, and for the single-quark form factors of the \eta_c and J/\psi.Comment: 7 pages, 5 postscript figures, contribution to the proceedings of QCHS7, Ponta Delgada, Sept. 200

Differences between heavy and light quarks

The quark Dyson-Schwinger equation shows that there are distinct differences between light and heavy quarks. The dynamical mass function of the light quarks is characterised by a sharp increase below 1 GeV, whereas the mass function of the heavy quarks is approximately constant in this infrared region. As a consequence, the heavy-meson masses increase linearly with the current-quark masses, whereas the light pseudoscalar meson masses are proportional to the square root of the current-quark masses.Comment: 4 pages, 3 figures, Contribution to the IVth International Workshop on Progress in Heavy Quark Physics, 20-22 Sept. 1997, Rostoc

Diquark properties and their role in hadrons

Diquark correlations are important in baryons, which can be modeled as quark-diquark bound states. In addition, diquarks could play a role in non-standard hadrons such as tetraquarks and pentaquarks. Here, we obtain properties of these diquarks from the corresponding bound state equation, using a model for the effective quark-quark interaction that has proved successful in the light meson sector. Subsequently, we use the same model to estimate the masses of the lightest diquark-diquark and diquark-antidiquark states.Comment: 3 pages, 2 .eps figures, contribution to the proceedings of the 19th European Conference on Few-Body Problems in Physics, Groningen, the Netherlands, Aug. 200

The quark-photon vertex and meson electromagnetic form factors

The ladder Bethe-Salpeter solution for the dressed photon-quark vertex is used to study the low-momentum behavior of the pion electromagnetic and the $\gamma^\star \pi^0 \gamma$ transition form factors. With model parameters previously fixed by light meson masses and decay constants, the low-momentum slope of both form factors is in excellent agreement with the data. In comparison, the often-used Ball-Chiu Ansatz for the vertex is found to be deficient; less than half of the obtained $r_\pi^2$ is generated by that Ansatz while the remainder of the charge radius could be attributed to the tail of the $\rho$ resonance.Comment: 4 pages, 2 figures, uses espcrc1.sty, talk presented at PANIC99, Uppsala, Swede

Facets of confinement and dynamical chiral symmetry breaking

The gap equation is a cornerstone in understanding dynamical chiral symmetry breaking and may also provide clues to confinement. A symmetry-preserving truncation of its kernel enables proofs of important results and the development of an efficacious phenomenology. We describe a model of the kernel that yields: a momentum-dependent dressed-quark propagator in fair agreement with quenched lattice-QCD results; and chiral limit values: f_pi= 68 MeV and = -(190 MeV)^3. It is compared with models inferred from studies of the gauge sector.Comment: 5 pages, 3 figures; contribution to the proceedings of Quark Nuclear Physics (QNP 2002), Juelich, Germany, 9-14 Jun 200