Virtual Compton Scattering measurements in the nucleon resonance region
2019
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 \( \alpha_E = (5.3 \pm 0.6_{stat} \pm 1.3_{sys}) 10^{-4}\) fm3 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.
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