ABSTRACT:We review the implications of the spontaneous chiral symmetry breaking in QCD for processes involving one, two or more nucleons.

We examine the meson-baryon interaction in the strangeness S = −1 sector using an effective chiral Lagrangian. Potentials are derived from this Lagrangian and used in a coupled-channel calculation of the low energy observables. The potentials are constructed such that in the Born approximation the s-wave scattering amplitude is the same as that given by the effective chiral Lagrangian, up to order q 2 . Comparison is made with the available low energy hadronic data of the coupled K − p, Σπ, Λπ system, which includes the Λ(1405) resonance, K − p elastic and inelastic scattering, and the threshold branching ratios of the K − p decay. Good fits to the experimental data and estimates of previously unknown Lagrangian parameters are obtained. *

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Within the one-loop approximation of baryon chiral perturbation theory we calculate all one-pion and two-pion exchange contributions to the nucleonnucleon interaction. In fact we construct the elastic NN-scattering amplitude up to and including third order in small momenta. The phase shifts with orbital angular momentum L ≥ 2 and the mixing angles with J ≥ 2 are given parameterfree and thus allow for a detailed test of chiral symmetry in the two-nucleon system. We find that for the D-waves the 2π-exchange corrections are too large as compared with empirical phase shifts, signaling the increasing importance of shorter range effects in lower partial waves. For higher partial waves, especially for G-waves, the model independent 2π-exchange corrections bring the chiral prediction close to empirical NN phase shifts. We propose to use the chiral NN phase shifts with L ≥ 3 as input in a future phase shift analysis. Furthermore, we compute the irreducible two-pion exchange NN-potentials in coordinate space. They turn out to be of van-der-Waals type, with exponential screening of two-pion mass range. * Work supported in part by BMBF and DFG.

Results for the ΛN and ΣN interactions obtained at next-to-leading order in chiral effective field theory are reported. At the order considered there are contributions from one-and two-pseudoscalar-meson exchange diagrams and from four-baryon contact terms without and with two derivatives. SU(3) flavor symmetry is imposed for constructing the hyperon-nucleon interaction while the explicit SU(3) symmetry breaking by the physical masses of the pseudoscalar mesons (π, K, η) is taken into account. An excellent description of the hyperon-nucleon system can be achieved at next-to-leading order. It is on the same level of quality as the one obtained by the most advanced phenomenological hyperon-nucleon interaction models.