Virtual Compton Scattering measurements in the nucleon resonance region

Abstract

We report on new measurements of the electric Generalized Polarizability (GP) of the proton $\hat{I}{\pm }_E$ in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have found that $\hat{I}{\pm }_E=(5.3\pm 0.6_{stat}\pm 1.3_{sys})10^{-4}f{m}^3$ at $Q^2=0.20(GeV/c{)}^2$. The new measurements, when considered along with the rest of the world data, suggest that $\hat{I}{\pm }_E$ 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 $N\to \hat{I}”$ transition through the exploration of the $p(e,e^{\prime }p){\hat{I}}^3$ 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.