J. E. F. T. Ribeiro

A low lying scalar meson nonet in a unitarized meson model

AbstractA unitarized non-relativistic meson model which is successful for the description of the heavy and light vector and pseudo-scalar mesons yields in its extension to the scalar mesons for the same model parameters a complete nonet below 1 GeV. In the unitarization scheme real and virtual meson meson decay channels are coupled to the quark antiquark confinement channels. The flavor dependent harmonic oscillator confining potential itself has bound states ɛ(1.3 GeV),S(1.5 GeV), δ(1.3 GeV), κ(1.4 GeV), similar to the results of other bound stateq

Confinement and parity doubling in heavy-light mesons

\bar q

Chiral symmetry restoration in excited hadrons, quantum fluctuations, and quasiclassics

models. However, the full coupled channel equations show poles at ɛ(0.5 GeV),S(0.99 GeV), δ(0.97 GeV), κ(0.73 GeV). Not only these pole positions can be calculated in our model also cross sections and phase shifts in the meson scattering channels which are in reasonable agreement with the available data for ππ, ηπ andKπ inS-wave scattering.

Chiral corrections to baryon properties with composite pions

The pi-pi scattering amplitude calculated with a model for the quark-antiquark interaction in the framework of the Covariant Spectator Theory (CST) is shown to satisfy the Adler zero constraint imposed by chiral symmetry. The CST formalism is established in Minkowski space and our calculations are performed in momentum space. We prove that the axial-vector Ward-Takahashi identity is satisfied by our model. Then we show that, similar to what happens within the Bethe-Salpeter formalism, application of the axial-vector Ward-Takahashi identity to the CST pi-pi scattering amplitude allows us to sum the intermediate quark-quark interactions to all orders. The Adler self-consistency zero for pi-pi scattering in the chiral limit emerges as the result for this sum.

Microscopic derivation of pion–nucleon and pion–delta scattering lengths

In this paper, we study the chiral symmetry restoration in the hadronic spectrum in the framework of generalised Nambu-Jona-Lasinio quark models with instantaneous confining quark kernels. We investigate a heavy-light quarkonium and derive its bound-state equation in the form of a Schroedingerlike equation and, after the exact inverse Foldy-Wouthuysen transformation, in the form of a Diraclike quation. We discuss the Lorentz nature of confinement for such a system and demonstrate explicitly the effective chiral symmetry restoration for highly excited states in the mesonic spectrum. We give an estimate for the scale of this restoration.

Field theory description of vacuum replicas

The resonating group method is used to study the effective potential between two nucleons, due to the confining potential between quarks taken together with the Pauli principle for quarks. The potential used has harmonic form, unitary spin factor λ(1)·λ(2), and both ordinary and spin-spin components. Its parameters are determined by appeal to experimental data, and phase shifts are calculated for the1S0 and3S1 states of the nuncleon-nucleon (NN) system. The results indicate that the repulsive core in theNN potential may arise from quark antisymmetrization required if nucleons are composed of quarks.

Quantifying zigzag motion of quarks

In this paper, we discuss the transition to the semiclassical regime in excited hadrons, and consequently, the restoration of chiral symmetry for these states. We use a generalised Nambu-Jona-Lasinio model with the interaction between quarks in the form of the 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. It has been shown recently that the effective single-quark potential is of the Lorentz-scalar nature, for the low-lying hadrons, while, for the high-lying states, it becomes a pure Lorentz vector and hence the model exhibits the restoration of chiral symmetry. We demonstrate explicitly the quantum nature of chiral symmetry breaking, the absence of chiral symmetry breaking in the classical limit as well as the transition to the semiclassical regime for excited states, where the effect of chiral symmetry breaking becomes only a small correction to the classical contributions.

Chiral symmetry and excited baryons

A calculational scheme is developed to evaluate chiral corrections to properties of composite baryons with composite pions. The composite baryons and pions are bound states derived from a microscopic chiral quark model. The model is amenable to standard many-body techniques such as the BCS and random phase approximation formalisms. An effective chiral model involving only hadronic degrees of freedom is derived from the macroscopic quark model by projection onto hadron states. Chiral loops are calculated using the effective hadronic Hamiltonian. A simple microscopic confining interaction is used to illustrate the derivation of the pion-nucleon form factor and the calculation of pionic self-energy corrections to the nucleon and D(1232) masses.