Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2

Abstract

We report new precise p$(\vec{e},e^{\prime }p){\pi }^0$ measurements at the peak of the ${\Delta }^{+}(1232)$ resonance at $Q^2=\unit[0.20](GeV/c)^2$ performed at the Mainz Microtron (MAMI). The new data are sensitive to both the electric quadrupole ($E2$) and the coulomb quadrupole ($C2$) amplitudes of the ${\gamma }^{\ast }N\to \Delta$ transition. They yield precise quadrupole to dipole amplitude ratios CMR $=(-5.09\pm 0.28_{stat+sys}\pm 0.30_{model})$ and EMR $=(-1.96\pm 0.68_{stat+sys}\pm 0.41_{model})$ for $M_{1+}^{3/2}=(39.57\pm 0.75_{stat+sys}\pm 0.40_{model})(10^{-3}/m_{{\pi }^{+}})$. 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.

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• $Q^2$ Dependence of Quadrupole Strength in the ${\gamma }^{\ast }p\to {\Delta }^{+}(1232)\to p{\pi }^0$ Transition
• Analysis of Pion Photoproduction Data
• Dynamical Study of the Delta Excitation in N(e,e' pi) Reactions
• Electroproduction of the Delta Resonance at High Momentum Transfer
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• Search for Quadrupole Strength in the Electro-excitation of the Delta(1232)
• Investigation of the conjectured nucleon deformation at low momentum transfer
• Measurement of the N to Delta(1232) Transition at High Momentum Transfer by pi0 Electroproduction
• Chiral Effective-Field Theory in the Delta(1232) Region: Pion Electroproduction on the Nucleon
• The N to Delta electromagnetic transition form factors from Lattice QCD
• Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the Δ(1232) resonance
• Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory
• Pi0 photo- and electroproduction at threshold within a dynamical model
• Recoil Polarization for Delta Excitation in Pion Electroproduction
• Measurement of the Polarized Structure Function ${\sigma }_{L{T}^{\prime }}$ for $p(\vec{e},e^{\prime }p){\pi }^o$ in the $\Delta (1232)$ Resonance Region
• Lowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution
• Measurement of the Polarized Structure Function ${\sigma }_{L{T}^{\prime }}$ for $p(\vec{e},e^{\prime }{\pi }^{+})n$ in the $\Delta (1232)$ Resonance Region
• Signatures of chiral dynamics in the Nucleon to Delta transition
• Measurement of the LT-asymmetry in π^0 electroproduction at the energy of the Δ(1232) resonance
• Measurement of the beam-helicity asymmetry in the p(e_pol,e'p)pi_0 reaction at the energy of the Delta(1232) resonance
• Measurement of the Transverse-Longitudinal Cross Sections in the p (e,e'p)pi0 Reaction in the Delta Region
• N to Delta electromagnetic transition form factors from Lattice QCD
• Deviation of the Nucleon Shape From Spherical Symmetry: Experimental Status
• Measurement of the R${}_{LT}$ response function for ${\pi }^0$ electroproduction at $Q^2$ = 0.070 (GeV/c)${}^2$ in the $N\to \Delta$ transition}
• Electromagnetic form factors in the Hypercentral Constituent Quark Model

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