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

We report new precise p (over(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. © 2007

Measurements of the γ* → Δ reaction at low Q2

We report new p(e, e′p)π̊ measurements in the Δ+(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 field theory 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. © Società Italiana di Fisica / Springer-Verlag 2013

Virtual Compton scattering measurements in the γ*N→Δ transition

We report on new H(e,e′p)γ measurements in the Δ(1232) resonance at Q2=0.06 (GeV/c)2 carried out simultaneously with H(e,e′p)π0. It is the lowest Q2 for which the virtual Compton scattering (VCS) reaction has been studied in the first resonance region. The VCS measured cross sections are well described by dispersion-relation calculations in which the multipole amplitudes derived from H(e,e′p)π0 data are used as input, thus confirming the compatibility of the results. The derived resonant magnetic dipole amplitude M1+3/2=(40.60±0.70stat+sys) (10-3/mπ+) at W=1232 MeV is in excellent agreement with the value extracted from H(e,e′p)π0 measurements. © 2008 The American Physical Society

Measurements of the γ*p→Δ reaction at low Q2: Probing the mesonic contribution

The determination of nonspherical angular momentum amplitudes in nucleons at long ranges (low Q2) was accomplished through the p(e→,e'p)π0 reaction in the Δ region at Q2=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 σLT' structure function. These results confirm the dominance, and general Q2 variation, of the pionic contribution at large distances. © 2008 The American Physical Society

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