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 $({M}_{1+})$, electric $({E}_{1+})$, and scalar $({S}_{1+})$ multipoles in the ${\ensuremath{\gamma}}^{*}p\ensuremath{\rightarrow}{\ensuremath{\Delta}}^{+}\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{0}$ transition. We report new experimental values for the ratios ${E}_{1+}{/M}_{1+}$ and ${S}_{1+}{/M}_{1+}$ over the range ${Q}^{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.4--1.8{\mathrm{GeV}}^{2}$, extracted from precision $p({e,e}^{\ensuremath{'}}p){\ensuremath{\pi}}^{0}$ data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.
Q2Dependence of Quadrupole Strength in theγ*p→Δ+(1232)→pπ0Transition | Litlas
