Deformation of the nucleon and nucleon-nucleon interaction in the quark model

The QCD property that must be included in effective models describing QCD at low energies is chiral symmetry. It is conserved if the quark masses are assumed to be zero. This symmetry is spontaneously broken, which leads to the constituent quark, and, as the Goldstone Boson, one obtains the pion and its chiral partner, the σ meson. We use the linear σ model, which has, in relation to the nonlinear one, the advantage that one treats pions π and σ mesons not only on the chiral circle; fluctuations around it are also allowed. The scale of these fluctuations is the σ-meson mass. If one eliminates gluons in the second order in a potential, 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. Within this model, we describe the photo- and electroexcitation 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 to the M1 transition from the nucleon to the delta resonance. The quadrupole contribution of this transition was described in the past by a D-wave admixture due to tensor forces from gluon and pion exchanges. This yields values that are more than a factor of 10 too small in relation to recent data on the C2/E2 Sachs transition form factor. We show that meson and gluon pair-exchange currents can explain data without a large nucleon or delta deformation. The same model is then used to describe nucleon-nucleon phase shifts. In order to achieve good agreement with data, it is necessary to include, to all orders, couplings to Δ channels. The 1S0 phase shift can be described in agreement with data only if the coupling to the 5D0 nucleon delta channel is taken into account