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

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 $P_{LL} -P_{TT} / epsilon$ and $P_{LT}$, as well as the electric and magnetic generalized polarizabilities $alpha_{E1}(Q^2)$ and $beta_{M1}(Q^2)$. These new results show a smooth and rapid fall-off of $alpha_{E1}(Q^2)$, in contrast to previous measurements at $Q^2$ = 0.33 GeV$^2$, and provide for the first time a precise mapping of $beta_{M1}(Q^2)$ in the low-$Q^2$ region.