Measurements of the gamma*p->Delta Reaction At Low Q^2: Probing the Mesonic Contribution

TL;DR

This study measures the gamma*p->Delta reaction at low Q^2 to investigate the mesonic contributions to nucleon structure, providing precise data that supports the dominance of pionic effects at large distances.

Contribution

It provides new experimental data on the gamma*p->Delta reaction at low Q^2, confirming the mesonic contribution's dominance and validating theoretical models with high accuracy.

Findings

Data agrees with lattice and effective field theory predictions.

Phenomenological models fit the experimental data well.

Results confirm the pionic contribution's dominance at large distances.

Abstract

The determination of non-spherical angular momentum amplitudes in nucleons at long ranges (low Q^{2}), was accomplished through the $p(\vec{e},e'p)\pi^0$ reaction in the Delta region at $Q^2=0.060$, 0.127, and 0.200 (GeV/c)^2 at the Mainz Microtron (MAMI) with an accuracy of 3%. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios have been obtained with an estimated model uncertainty which 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 $\sigma_{TL'}$ structure function. These results confirm the dominance, and general Q^2 variation, of the pionic contribution at large distances.