Deviation of the Nucleon Shape From Spherical Symmetry: Experimental Status

Abstract

In this brief pedagogical overview the physical basis of the deviation of the nucleon shape from spherical symmetry will be presented along with the experimental methods used to determine it by the gamma* p -> Delta reaction.The fact that significant non-spherical electric(E2) and Coulomb quadrupole(C2) amplitudes have been observed will be demonstrated. These multipoles for the N,Delta system as a function of Q^2 from the photon point through 4 GeV^2 have been measured with modest precision. Their precise magnitude remains model dependent due to the contributions of the background amplitudes, although rapid progress is being made to reduce these uncertainties. A discussion of what is required to perform a model independent analysis is presented. All of the data to date are consistent with a prolate shape for the proton (larger at the poles) and an oblate shape(flatter at the poles) for the Delta. It is suggested here that the fundamental reason for this lies in the spontaneous breaking of chiral symmetry in QCD and the resulting, long range(low Q^2), effects of the pion cloud. This verification of this suggestion, as well as a more accurate measurement of the deviation from spherical symmetry, requires further experimental and theoretical effort.

8 pages, 8 figures, enhanced conference proceedings

References in corpus (5)

• $Q^2$ Dependence of Quadrupole Strength in the ${\gamma }^{\ast }p\to {\Delta }^{+}(1232)\to p{\pi }^0$ Transition
• Dynamical Study of the Delta Excitation in N(e,e' pi) Reactions
• Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the Δ(1232) resonance
• Intrinsic quadrupole moment of the nucleon
• Measurement of the beam-helicity asymmetry in the p(e_pol,e'p)pi_0 reaction at the energy of the Delta(1232) resonance

Cited by in corpus (25)

• Investigation of the conjectured nucleon deformation at low momentum transfer
• 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
• Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory
• D-state effects in the electromagnetic N-Delta transition
• Lowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution
• Electromagnetic N->Delta transition and neutron form factors
• Signatures of chiral dynamics in the Nucleon to Delta transition
• Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2
• N to Delta electromagnetic transition form factors from Lattice QCD
• Measurements of the gamma*p->Delta Reaction At Low Q^2: Probing the Mesonic Contribution
• Electromagnetic Transition Form Factors of Baryon Resonances
• Quadrupole moments of low lying baryons with spin 1/2^+, spin 3/2^+, and spin 3/2^+ \to 1/2^+ transitions
• Electroexcitation of the ${\Delta }^{+}(1232)$ at low momentum transfer
• Measurements of the γ* p --> Δ(1232) reaction at low Q2
• Hints on the quadrupole deformation of the $\Delta$(1232)
• New low-$Q^2$ measurements of the ${\gamma }^{\ast }N\to \Delta (1232)$ Coulomb quadrupole form factor, pion cloud parametrizations and Siegert's theorem
• D-state configurations in the electromagnetic form factors of the nucleon and the Delta(1232) resonance
• Nucleon deformation and atomic spectroscopy
• Analysis of the quadrupole deformation of $\Delta$(1232) within an effective Lagrangian model for pion photoproduction from the nucleon
• Combined parametrization of the neutron electric form factor and the ${\gamma }^{\ast }N\to \Delta (1232)$ quadrupole form factors
• Sea contribution to the charge radii and quadrupole moment of $J^P={\frac{1}{2}}^{+},{\frac{3}{2}}^{+}$ baryons
• Symmetry/asymmetry within a cylindrical lattice model of nuclear structure predicts cosmic abundance/scarcity
• Pion electroproduction measurements in the nucleon resonance region
• Colour Confinement and Deformed Baryons in Quantum Chromodynamics