Measurements of the \gamma * p --> \Delta(1232) reaction at low Q2

TL;DR

This study presents new low-Q2 measurements of the (1232) resonance reaction, testing chiral effective theories and highlighting discrepancies with constituent quark models, thus guiding future theoretical improvements.

Contribution

It provides novel low-Q2 experimental data on the (1232) resonance, challenging existing models and supporting dynamical and chiral effective calculations.

Findings

Results disagree with constituent quark models

Data agree with pion cloud and chiral calculations

Highlights need for theoretical refinement

Abstract

We report new p$(\vec{e},e^\prime p)\pi^\circ$ measurements in the $\Delta^{+}(1232)$ resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.