Virtual Compton Scattering measurements in the nucleon resonance region

Abstract

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.