Models of baryon structure predict a small quadrupole deformation of the nucleon due to residual tensor forces between quarks or distortions from the pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through the Q2 dependence of the magnetic (M1+), electric (E1+), and scalar (S1+) multipoles in the γ∗p→Δ+→pπ0 transition. We report new experimental values for the ratios E1+/M1+ and S1+/M1+ over the range Q2=0.4–1.8GeV2, extracted from precision p(e,e′p)π0 data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.
Physical Review Letters
Joo, K.; Biselli, Angela; and CLAS Collaboration, "Q2 Dependence of Quadrupole Strength in the γ*p→Δ+(1232)→pπ0 Transition" (2002). Physics Faculty Publications. 103.
K. Joo et al. [CLAS Collaboration], "Q2 Dependence of Quadrupole Strength in the γ*p→Δ+(1232)→pπ0 Transition", Physical Review Letters 88.12 (2002) DOI: 10.1103/PhysRevLett.88.122001