On Goldstone bosons decoupling from high-lying hadrons

In this paper, we discuss a decoupling of the Goldstone bosons from highly excited hadrons in relation to the restoration of chiral symmetry in such hadrons. We use a generalised Nambu-Jona-Lasinio model with the interaction between quarks in the form of an instantaneous Lorentz-vector confining potential. This model is known to provide spontaneous breaking of chiral symmetry in the vacuum via the standard selfenergy loops for valence quarks. For highly excited hadrons, where the typical momentum of valence quarks is large, the loop contributions represent only a small correction to the chiral-invariant classical contributions and asymptotically vanish. Consequently the chiral symmetry violating Lorentz-scalar dynamical mass of quarks vanishes. Then the conservation of the axial vector current in the chiral limit requires, via the Goldberger-Treiman relation, that the valence quarks decouple from the Goldstone boson. As a consequence, the whole hadron decouples from the Goldstone boson as well, which implies that its axial constant also vanishes.Comment: RevTeX4, 21 pages, 5 Postscript figures, uses epsfig.sty, new chapter with numerical estimates added, to appear in Phys.Rev.

Generalized Weinberg's approach to the a0/f0 case

The problem of whether it is possible to distinguish composite from elementary particles is studied in the framework of Weinberg's approach. The possibility to extend this approach to the case of unstable particles in the presence of inelastic channels is considered. The interplay between the low-energy scattering data and the admixture of a bare state in the resonance is discussed, and the implications for the a0(980)/f0(980) case are outlined.Comment: LaTeX2e, 5 pages, 2 figures, to appear in the proceedings of the HADRON03 conference, Aschaffenburg, Germany. September 2003, typo correcte

On two- and three-body descriptions of hybrid mesons

Hybrid mesons are exotic mesons in which the color field is not in its ground state. Their understanding deserves interest from a theoretical point of view, because it is intimately related to nonperturbative aspects of QCD. In this work, we analyze and compare two different descriptions of hybrid mesons, namely a two-body $q\bar q$ system with an excited string, or a three-body $q\bar q g$ system. In particular, we show that the constituent gluon approach is equivalent to an effective excited string in the heavy hybrid sector. Instead of a numerical resolution, we use the auxiliary field technique. It allows to find simplified analytical mass spectra and wave functions, and still leads to reliable qualitative predictions. We also investigate the light hybrid sector, and found a mass for the lightest hybrid meson which is in satisfactory agreement with lattice QCD and other effective models.Comment: 2 figure

Microscopic derivation of the pion coupling to heavy-light mesons

The Goldberger--Treiman relation for heavy--light systems is derived in the context of a quark model. As a paradigmatic example, the case of ${\bar D}\to {\bar D}' \pi$ is studied in detail. The fundamental role played by the pion two-component wave function, in the context of the Salpeter equation, is emphasized.Comment: 6 pages, 2 figures, version to appear in Phys. Rev.

Light-Quark Vector Meson Spectroscopy

The current situation for vector meson spectroscopy is outlined, and it is shown that the data are inconsistent with the generally-accepted model for meson decay. A possible resolution in terms of exotic mesons is given. Although this resolves some of the issues, fresh theoretical questions are raised.Comment: Plenary talk at HADRON2001, Protvin

QCD string model for hybrid adiabatic potentials

Hybrid adiabatic potentials are considered in the framework of the QCD string model. The einbein field formalism is applied to obtain the large-distance behaviour of adiabatic potentials. The calculated excitation curves are shown to be the result of interplay between potential-type longitudinal and string-type transverse vibrations. The results are compared with recent lattice data.Comment: LATEX2e, 10 pages, 4 PS figures. Plenary talk at HADRON2001, Protvino. Comment adde