Generalized dipole polarizabilities of the pion

Abstract

We discuss the virtual Compton scattering amplitude $γ^\ast+π\to γ+π$ which enters the reaction $\ddot{I}{€}^{-}+e^{-}\to \ddot{I}{€}^{-}+e^{-}+{\hat{I}}^3$. The low-energy scattering amplitude is divided into a model-independent part, involving only the electromagnetic form factor, and a residual part which contains structure information related specifically to (virtual) Compton scattering. In the limit of vanishing final-photon energy, the residual amplitude can be expressed in terms of three generalized dipole polarizabilities which are functions of the squared virtual-photon four-momentum. We study the VCS amplitude in the framework of chiral perturbation theory at $\mathcal{O}(p^4)$. At the one-loop level the generalized dipole polarizabilities are degenerate $\hat{I}{\pm }_L(q^2)=\hat{I}{\pm }_T(q^2)=-{\hat{I}}^2(q^2)$.

11 pages, Latex2e, 5 postscript figures, uses ws-p8-50x6-00.cls, invited talk at the 2nd International Conference on Perspectives in Hadronic Physics, 10 - 14 May 1999, Trieste, Italy