Determination of quadrupole strengths in the γ∗p→Δ(1232) transition at Q2=0.20(GeV/c)2

We report new precise p(e→,e′p)π0 measurements at the peak of the Δ+(1232) resonance at Q2=0.20(GeV/c)2 performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric (E2) and the Coulomb (C2) quadrupole amplitudes of the γ∗N→Δ transition. They yield precise quadrupole to dipole amplitude ratios: CMR=(−5.09±0.28stat+sys±0.30model)% and EMR=(−1.96±0.68stat+sys±0.41model)% for M1+3/2=(39.57±0.75stat+sys±0.40model)(10−3/mπ+). The new results are in disagreement with Constituent Quark Model predictions and in qualitative agreement with models that account for mesonic contributions, including recent Lattice calculations. They thus give further credence to the conjecture of deformation in hadronic systems favoring the attribution of the origin of deformation to the dominance of mesonic effects.