The χ-BS(3) approach

where the parameter f fπ 92.4 MeV is known from the decay process of charged pions. In contrast to the successful application of the chiral Lagrangian in the flavor SU(2) sector, its application to the strange sector of QCD is flawed by a number of subtleties if the machinery of chiral perturbation theory is applied. The leading interaction term fails miserably in describing the s-wave scattering lengths of both kaons and antikaons off a nucleon. Most stunning is the failure of reproducing the repulsive K−p scattering length [2]. The chiral Lagrangian predicts an attractive scattering length at leading order instead. This is closely linked to the presence of the Λ(1405) resonance in the K−p scattering amplitude just below the K−p threshold. Considerable theoretical progress has been made over the last few years by incorporating the dynamics of that Λ(1405) resonance into the chiral dynamics [3, 4]. These early works consider s-wave interactions to leading [4] and subleading [3] chiral order only. The key point is to change approximation strategy and expand the interaction kernel rather than the scattering amplitude directly. That amounts to solving some type of coupled-channel scattering equation like the Lippmann-Schwinger or the BetheSalpeter equation. As a consequence the Λ(1405) resonance is generated dynamically by coupled channel effects. The advantage of the chiral approach over previous phenomenological K-matrix analyses (see e.g. [5]), which also lead to a dynamic generation of the Λ(1405) resonance, is the systematic parameter reduction implied by the chiral SU(3) symmetry of QCD. A realistic description of the antikaon-nucleon scattering process requires the inclusion of all SU(3) channels K̄N, πΛ, πΣ, ηΣ, ηΛ and KΞ together with correction terms predicted by the chiral SU(3) Lagrangian. In the χ-BS(3) approach [1], which considers spand d-wave interactions to subsubleading order, the number of free parameters controlling the chiral correction terms is significantly reduced by insisting on sum rule relations as they arise from QCD in the large-Nc limit. It is demonstrated for the first time that the chiral SU(3) Lagrangian does describe all low-energy pion-, kaonand antikaon-nucleon scattering data fairly well, once chiral perturbation theory is applied to the covariant scattering kernel of the BetheSalpeter scattering equation. 0 20 40 60 80 100