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 ${\alpha }_E=(5.3\pm 0.6_{stat}\pm 1.3_{sys})10^{-4}f{m}^3$ at $Q^2=0.20(GeV/c{)}^2$. The new measurements, when considered along with the rest of the world data, suggest that ${\alpha }_E$ 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 $N\to \Delta$ transition through the exploration of the $p(e,e^{\prime }p)\gamma$ 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.

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• The Present and Future of QCD
• Measurement of the neutron charge radius and the role of its constituents
• Measured proton electromagnetic structure deviates from theoretical predictions
• Virtual Compton Scattering and Nucleon Generalized Polarizabilities
• New Insight in the $Q^2$-Dependence of Proton Generalized Polarizabilities
• Measurement of the Generalized Polarizabilities of the Proton at Intermediate $Q^2$
• Pion electroproduction measurements in the nucleon resonance region
• The Proton Electromagnetic Generalized Polarizabilities