Subnuclear degrees of freedom in photoabsorption and scattering

This paper concentrates on the study of subnuclear degrees of freedom in nuclei using the electromagnetic probes of photoabsorption and photon scattering. In these processes subnuclear degrees of freedom manifest themselves by the presence of exchange operators like exchange current and exchange two-photon operators, which effectively describe underlying hidden degrees of freedom. The photon scattering amplitude is discussed where the specific forms of resonance and two-photon scattering amplitudes are given. The derivation of the low energy expansion with the aid of the gauge conditions is sketched and optical theorem and dispersion relations are briefly mentioned. The generalized polarizabilities are derived and the decomposition of the low energy expansion of the scattering amplitude in polaizabilities is given in detail. The explicit forms of pi-exchange current and TPA are given and the construction of a consistent exchange current for the paris potential is outlined. Exchange contributions to various electromagnetic processes using a deuteron target are discussed. Real delta excitation in photon scattering is considered and nuclear medium effects with nucleonic contributions are examined