Measurements of theγ*p→Δreaction at lowQ2: Probing the mesonic contribution

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Abstract

The determination of nonspherical angular momentum amplitudes in nucleons at long ranges (low ${Q}^{2}$) was accomplished through the $p(\stackrel{\ensuremath{\rightarrow}}{e},{e}^{'}p){\ensuremath{\pi}}^{0}$ reaction in the \ensuremath{\Delta} region at ${Q}^{2}=0.060$, 0.127, and 0.200 (GeV/$c){}^{2}$ at the Mainz Microtron with an accuracy for the cross sections of 4%. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios have been obtained with an estimated model uncertainty that is approximately the same as the experimental uncertainty. Lattice and effective field theory predictions agree with our data within the relatively large estimated theoretical uncertainties. Phenomenological models are in good agreement with experiment when the resonant amplitudes are adjusted to the data. To check reaction model calculations additional data were taken for center-of-mass energies below resonance and for the ${\ensuremath{\sigma}}_{{\mathit{LT}}^{'}}$ structure function. These results confirm the dominance, and general ${Q}^{2}$ variation, of the pionic contribution at large distances.

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