Quark contribution to the nucleon polarizabilities and three-body forces
  We study the response of the nucleon, as a system of three bound (constituent) non relativistic quarks, to external (quasi static) electric and magnetic fields. The approach, based on a sum rule technique, is applied to a large class of two and three-body interquark potentials. Lower and upper bounds to the electric polarizability and para-magnetic susceptibility are explicitly calculated within a large variety of constituent models and their values related to the features of the interquark interaction picture. The r\^ole of three-body forces is investigated in details as well as the effects of SU(6) breaking terms in the potential model. Our results can be used to extract the mesonic contributions to the static polarizabilty and susceptibility. The quark degrees of freedom give a quite sizeable contributions to both and the meson cloud accounts roughly for 30% and 60% of the electric proton and neutron polarizability respectively. The quark contribution to the paramagnetic susceptibility is even higher and the mesonic effects are rather uncertain. 
Elvira Biasioli, Marco Traini, Renzo Leonardi