Structure of the nucleon and spin-polarizabilities

Abstract

Spin-polarizabilities are predicted by calculating the cross-section difference $\sigma_{3/2}-\sigma_{1/2}$ from available data for the resonance couplings $A_{3/2}$ and $A_{1/2}$ and CGLN amplitudes. The forward spin-polarizabilities are predicted to be $\gamma^{(p)}_0=-0.58\pm 0.20$ and $\gamma^{(n)}_0=+0.38\pm 0.22$ in units of $10^{-4}$fm$^4$ where the different signs are found to be due to the isospin dependencies of the $E_{0+}$ and the $(M,E)^{(1/2)}_{1+}$ amplitudes. The backward spin-polarizabilities are predicted to be $\gamma^{(p)}_\pi=-36.6$ and $\gamma^{(n)}_\pi=+58.3$, to be compared with the experimental values $\gamma^{(p)}=-36.4\pm 1.5$ and $\gamma^{(n)}_\pi=+58.6\pm 4.0$. Electric $\gamma_E$ and magnetic $\gamma_M$ spin-polarizabilities are introduced and discussed in terms of the $E1$ and $M1/E2$ components of the photo-absorption cross section of the nucleon.