Polarizabilities of the nucleon and spin dependent photo-absorption

The polarizabilities α (electric), β (magnetic) and γπ (backward spin) of the nucleon are investigated in terms of the degrees of freedom of the nucleon using recent results for the CGLN amplitudes and resonance couplings A1/2 and A3/2. The photon excitation strengths of the excited states are given in terms of partial integrated photo-absorption cross sections and resonant contributions of the Gerasimov–Drell–Hearn (GDH) sum rule. As a test of the present predictions, cross section differences (σ3/2−σ1/2) of the excited states are compared with data recently measured at MAMI (Mainz) and ELSA (Bonn). In order to explain differences between proton and neutron, radiative widths of the excited states are compared with nonrelativistic and relativistic predictions based on the SU(6) harmonic oscillator (HO) quark model. A complete list of partial contributions from the s-channel and the t-channel are given for the polarizabilities.