Virtual Compton Scattering measurements in the nucleon resonance region
Résumé
We report on new measurements of the electric Generalized Polarizability (GP) of the proton $ \alpha_{E}$ in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have found that $ \alpha_E = (5.3 \pm 0.6_{stat} \pm 1.3_{sys}) 10^{-4}$ fm$^{3}$ at $ Q^{2}=0.20$ (GeV/c)$^{2}$. The new measurements, when considered along with the rest of the world data, suggest that $ \alpha_E$ can be described by either a local plateau or by an enhancement in the region $ Q^2=0.20$ (GeV/c)$^{2}$ to 0.33 (GeV/c)$^{2}$. The experiment also provides the first measurement of the Coulomb quadrupole amplitude in the $ N \rightarrow \Delta$ transition through the exploration of the $ p(e,e'p)\gamma$ reaction. The new measurement gives $ CMR = (-4.4 \pm 0.8_{stat} \pm 0.6_{sys})\%$ at $ Q^2=0.20$ (GeV/c)$^{2}$ and is consistent with the results from the pion electroproduction world data. It has been obtained using a completely different extraction method, and therefore represents a strong validation test of the world data model uncertainties.