Q2 dependence of quadrupole strength in the γ*p→Δ+(1232)→pπ0 transition -: art. no. 122001

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 Q(2) dependence of the magnetic (M1+), electric (E1+), and scalar (S1+) multipoles in the gamma* p --> Delta(+) --> ppi(0) transition. We report new experimental values for the ratios E1+/M1+ and S1+/M1+ over the range Q(2) = 0.4-1.8 GeV2, extracted from precision p(e,e'p)pi(0) data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.