Quark model, chiral symmetry deformation of the nucleon and the nucleon-nucleon interaction

A property of Quantum Chromodynamics (QCD) which should be included into effective models describing QCD at low energies is chiral symmetry. It is conserved if one: assumes that the quark masses are zero. This symmetry is spontaneously broken, which leads to constituent quark and as the Goldstone Boson one obtains the pion and its chiral partner the sigma meson. We use the linear sigma model which has compared to the non-linear one the advantage, that one treats pions pi and sigma mesons not only on the chiral circle, but allows also fluctuations around it. The scale of this fluctuations is the sigma meson mass. If one eliminates gluons in second order, one obtains the "Tuebingen chiral quark model" with effective gluon exchange between the quarks and with pions and sigma mesons. A confinement potential is added. With this model we describe the photo and electro-excitation of the nucleon into the delta resonance and the decay of this resonance into a nucleon and a pion. The angular distribution gives information about the C2/E2 admixture into the M1 transition from the nucleon to the delta resonance. The quadrupole contribution of this transition has been described in the past by d state admixture due to tensor forces from the gluon and the pion exchange. This yields values which are more than a factor 10 too small compared with recent data for the C2/E2 Sachs transition form factor. Mie show that meson and gluon pair exchange currents can explain the data without the need of a large nucleon or delta deformation. The same model is then used to describe the nucleon-nucleon phase shifts. An essential ingredient for the good agreement is to include to all orders couplings to Delta channels. The S-1(0) phase shift call only be described in agreement with the data if the coupling to the D-5(0) nucleon delta channel is included.