Narrow Results

We discuss several aspects of the Λ(1405) resonance in relation to the recent theoretical developments in chiral dynamics. We derive an effective single-channel interaction based on chiral SU(3) coupled-channel approach, emphasizing the important role of the πΣ channel and the structure of the Λ(1405) in phenomenology. In order to clarify the structure of the resonance, we study the behavior with the number of colors (N_c) of the poles associated with the Λ(1405), and argue the physical meaning of the renormalization procedure.

Confinement and spontaneous breaking of chiral symmetry are assumed to generate the governing degrees of freedom of low-energy quantum chromodynamics. On this basis a relativistic constituent-quark model is constructed and formulated along an invariant mass operator within Poincaré-invariant quantum mechanics. The model is effectively applied to the spectroscopy of all known baryons of flavors u, d, s, c and b. The mass-operator eigenstates are furthermore tested with regard to the baryon electromagnetic and axial form factors. Through using the point form of relativistic quantum mechanics, these observables are obtained in a manifestly covariant manner. For all light and strange baryon ground states the electroweak structures are reproduced either in good agreement with phenomenology or, if no experimental data exist, in consistency with results available from lattice quantum chromodynamics. It is concluded that the relativistic constituent-quark model, relying on {QQQ} Fock states only, provides a universal framework for the description of low-energy baryons. The most important ingredients are spontaneous chiral-symmetry breaking and strict relativistic invariance.

A collective treatment of the I=0 scalar mesons below 2 GeV [σ(550), f₀(980), f₀(1370), f₀(1500) and f₀(1710)] in a nonlinear chiral Lagrangian framework that is constrained by the mass and the partial decay widths of the I=1/2,1 scalars [κ(900), , a₀(980) and a₀(1450)] is presented. The sub-structure of these states in terms of two and four quark components, as well as a glueball component is explored, and its correlation with the mass of f₀(1370) is studied. Consistency with the available experimental data suggests that the σ(550) is dominantly a nonstrange four-quark state, whereas the sub-structure of other I=0 states are sensitive to the input mass of f₀(1370). This investigation estimates the scalar glueball mass in the range 1.47–1.64 GeV.

Identifying a zero-range exchange of vector mesons as the driving force for the s-wave scattering of pseudo-scalar mesons off the baryon ground states, a rich spectrum of hadronic nuclei is formed. We argue that chiral symmetry and large-N_c considerations determine that part of the interaction which generates the spectrum. We suggest the existence of strongly bound crypto-exotic baryons, which contain a charm-anti-charm pair. Such states are narrow since they can decay only via OZI-violating processes.

Within the effective quark model with chiral U(3) × U(3) symmetry we calculate the S-wave and P-wave amplitudes and the partial widths of the nonleptonic decays of the Λ⁰-hyperon, Λ⁰ → pπ^- and Λ⁰ → nπ⁰. The theoretical results agree well with the experimental data. The angular distributions of the decay rates in dependence on the polarizations of baryons are analyzed both in the laboratory frame and in the rest frame of the Λ⁰-hyperon.

We investigate the and coupled channels system in a finite volume and study the properties of the Λ(1405) resonance. We calculate the energy levels in a finite volume and solve the inverse problem of determining the resonance position in the infinite volume. We devise the best strategy of analysis to obtain the two poles of the Λ(1405) in the infinite volume case, with sufficient precision to distinguish them.

The chiral next-to-next-to-next-to leading order nuclear forces¹⁻³ are used to obtain predictions for spin observables in elastic nucleon-deuteron scattering at E=13 MeV. The three-nucleon force is taken into account with all its complexity, including the short-range part and relativistic corrections. Presented examples of the polarization observables for elastic nucleon-deuteron scattering show visible contributions from these new structures in the three-nucleon potential which emerge for the first time at the next-to-next-to-next-to leading order. However, our results suggest that some modifications of the currently used model of the nuclear forces are necessary.

A relativistic nuclear energy density functional is developed, guided by two important features that establish connections with chiral dynamics and the symmetry breaking pattern of low-energy QCD: a) strong scalar and vector fields related to in-medium changes of QCD vacuum condensates; b) the long- and intermediate-range interactions generated by one-and two-pion exchange, derived from in-medium chiral perturbation theory, with explicit inclusion of Δ(1232) excitations. The results are at the same level of quantitative comparison with data as the best phenomenological relativistic mean-field models.

We present preliminary updates of results for QCD low energy constants.

We summarize the findings of our Working Group, which discussed progress in the understanding of Chiral Dynamics in the A = 2, 3, and 4 systems over the last three years. We also identify key unresolved theoretical and experimental questions in this field.

We study the structure of the σ meson, the lowest-lying resonance of the ππ scattering in the scalar-isoscalar channel, through the softening phenomena associated with the partial restoration of chiral symmetry. We build dynamical chiral models to describe the ππ scattering amplitude, in which the σ meson is described either as the chiral partner of the pion or as the dynamically generated resonance through the ππ attraction. It is shown that the internal structure is reflected in the softening phenomena; the softening pattern of the dynamically generated σ meson is qualitatively different from the behavior of the chiral partner of the pion. On the other hand, in the symmetry restoration limit, the dynamically generated σ meson behaves similarly to the chiral partner.