A combined chiral and 1/Nc expansion of the Bethe-Salpeter interaction kernel leads to a good description of the kaon-nucleon, antikaon-nucleon and pion-nucleon scattering data typically up to laboratory momenta of p lab 500 MeV. The covariant on-shell reduced coupled channel Bethe-Salpeter equation with the interaction kernel truncated to chiral order Q 3 and to the leading order in the 1/Nc expansion is evaluated.We review the recent application of the chiral SU(3) Lagrangian to mesonbaryon scattering 1 . The acronym 'χ-BS(3)' is used as to indicate that the Bethe-Salpeter scattering equation is applied and properly furnished with an interaction kernel constrained by the chiral SU(3) symmetry. In addition we consider the number of colors (N c ) in QCD as a large parameter relying on a systematic expansion of the interaction kernel in powers of 1/N c . Since the baryon octet and decuplet states are degenerate in the large-N c limit of QCD the latter are included as explicit degrees of freedom in our scheme. The coupled-channel Bethe-Salpeter kernel is evaluated in a combined chiral and 1/N c expansion including terms of chiral order Q 3 .In contrast to previous coupled channel approaches 2,3 that are based on the chiral Lagrangian, particular emphasis is put on the interplay of the regularization and renormalization of the scattering kernel and scattering amplitude. The use of phenomenological form factors or cutoff parameters is avoided. An important ingredient of the χ-BS(3) scheme is a systematic and covariant on-shell reduction of the Bethe-Salpeter equation. This is required as to avoid an unphysical and uncontrolled dependence of the scattering amplitudes on the choice of chiral coordinates or the choice of interpolating fields. Given any scheme the on-shell scattering amplitude should not change if a different representation of the chiral Lagrangian is used. In the χ-BS(3) scheme the on-shell reduction is implied unambiguously by the existence of a unique and covariant projector algebra which solves the Bethe-Salpeter equation for any choice of quasi-local interaction terms. The covariant projector algebra permits the application of dimensional regularization. In the language of the N/D method introduced by Chew and Mandelstam 4 this leads to a strong correlation of the many subtraction constants, which arise when imposing a

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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 molecules is formed. We argue that chiral symmetry and large-Nc 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. A narrow nucleon resonance is found at mass 3.52 GeV. It is a coupled-channel bound state of the (ηc N ), (D Σc) system, which decays dominantly into the (η ′ N ) channel. Furthermore two isospin singlet hyperon states at mass 3.23 GeV and 3.58 GeV are observed as a consequence of coupled-channel interactions of the (Ds Λc), (D Ξc) and (ηc Λ), (D Ξ ′ c ) states. Most striking is the small width of about 1 MeV of the lower state. The upper state may be significantly broader due to a strong coupling to the (η ′ Λ) state. The spectrum of crypto-exotic charm-zero states is completed with an isospin triplet state at 3.93 GeV and an isospin doublet state at 3.80 GeV. The dominant decay modes involve again the η ′ meson.

Abstract. Pion-nucleon scattering, pion photoproduction, and nucleon Compton scattering are analyzed within a scheme based on the chiral Lagrangian. Partial-wave amplitudes are obtained by an analytic extrapolation of subthreshold reaction amplitudes computed in chiral perturbation theory, where the constraints set by electromagnetic-gauge invariance, causality and unitarity are used to stabilize the extrapolation. Experimental data are reproduced up to energies √ s 1300 MeV in terms of the parameters relevant at order Q 3 . A striking puzzle caused by an old photon asymmetry measurement close to the pion production threshold is discussed.

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