New Insight in the $Q^2$-Dependence of Proton Generalized Polarizabilities

Abstract

Virtual Compton scattering on the proton has been investigated at three yet unexplored values of the four-momentum transfer $Q^2$: 0.10, 0.20 and 0.45 GeV${}^2$, at the Mainz Microtron. Fits performed using either the low-energy theorem or dispersion relations allowed the extraction of the structure functions $P_{LL}-P_{TT}/\hat{I}\mu$ and $P_{LT}$, as well as the electric and magnetic generalized polarizabilities $\hat{I}{\pm }_{E1}(Q^2)$ and ${\hat{I}}_{M1}^2(Q^2)$. These new results show a smooth and rapid fall-off of $\hat{I}{\pm }_{E1}(Q^2)$, in contrast to previous measurements at $Q^2$ = 0.33 GeV${}^2$, and provide for the first time a precise mapping of ${\hat{I}}_{M1}^2(Q^2)$ in the low-$Q^2$ region.

5 pages, 1 figure .eps. Version accepted by PRL