Nucleon polarizability: Recent progress and future plans

The electric and magnetic polarizabilities of the nucleon are fundamental structure constants which characterize the response of the constituents to external electric and magnetic fields. The polarizabilities therefore contain basic information about the internal structure of the system and can be used to test various QCD-based models of the nucleon. Much progress has been made in recent years on the study of nucleon structure via the electric and magnetic polarizabilities. For the proton, these ,values have been obtained exclusively through Compton scattering. Since 1990, new measurements at Illinois, Mainz and Saskatoon have made considerable improvements in both statistical and systematic precision. Further experiments will soon be running at Lund and Mainz. For the neutron, the only polarizability measurement to yield a definite result was performed at Oak Ridge by neutron scattering on a heavy nucleus. There are currently plans at Saskatoon and LEGS to extract these values by quasi-elastic and elastic photon scattering on deuterium. Theoretical treatments of the polarizabilities in quark models and chiral perturbation theory are generally consistent with the experimentally determined values and are essential for understanding the various contributions to the polarizability values. New measurements proposed for CEBAF will examine the virtual Compton scattering reaction (e, eprimey) in order to determine the Q2 dependence of the nucleon polarizability. The current status of experimental and theoretical work on nucleon polarizability will be reviewed, and plans for future experiments will be discussed