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 ${\gamma }_0^{(p)}=-0.58\pm 0.20$ and ${\gamma }_0^{(n)}=+0.38\pm 0.22$ 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 $(M,E{)}_{1+}^{(1/2)}$ amplitudes. The backward spin-polarizabilities are predicted to be ${\gamma }_{\pi }^{(p)}=-36.6$ and ${\gamma }_{\pi }^{(n)}=+58.3$, to be compared with the experimental values ${\gamma }^{(p)}=-36.4\pm 1.5$ and ${\gamma }_{\pi }^{(n)}=+58.6\pm 4.0$. 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.

Comments: 20 pages, 5 figures, 6 tables