RPA Description of the Electric Polarizability of the Nucleon

Abstract

Excited states of the nucleon are described as RPA configurations on a mean--field ground state taken from the MIT bag model. A residual interaction of a structure as in the Nambu--Jona--Lasinio model is used. The particle-hole states are coupled to good total angular momentum and isospin. Valence excitations of particle-hole type and quark--antiquark ($q\overline{q}$) states from the Dirac--sea are included. The dependence of the baryon spectrum and multipole response functions on the coupling constant $G$ is studied. At critical values of $G$ the 3q--ground state becomes degenerate with strongly collective $q\overline{q}$ modes. The model is used to calculate the electric polarizability of the neutron ${\alpha }_N$. Without residual interaction ${\alpha }_N=7\cdot 10^{-4}f{m}^3$ is found. With residual interaction the value increases to ${\alpha }_N=(7-11)\cdot 10^{-4}f{m}^3$. The measured value of ${\alpha }_N$ is reproduced within experimental error bars.

Comments: LaTeX, 19 pages, 2 figures