Evaluation of the E2/M1 Ratio in the N→Δ(1232) Transition from The

The transition from the nucleon to the $\mathrm{\ensuremath{\Delta}}(1232)$ resonance is a sensitive test for models of the nucleon structure. A magnetic dipole ($M1$) quark spin-flip transition essentially dominates photoexcitation of the $\mathrm{\ensuremath{\Delta}}$, but smaller components in the nucleon and $\mathrm{\ensuremath{\Delta}}$ wave functions allow also electric quadrupole ($E2$) contributions. The ratio $E2/M1$ then provides fundamental information on both the spatial deformation of the nucleon or $\mathrm{\ensuremath{\Delta}}$, and on the corresponding $D$ states in their quark-model wave functions. The authors measured the $E2/M1$ ratio via single ${\ensuremath{\pi}}^{0}$ production from the proton with a circularly polarized photon beam and a longitudinally polarized proton target, exploiting the presence of interference terms between the measured amplitudes that enhance the effect of smaller contributions. This most precise experimental result to date for the $E2/M1$ ratio gives deep insight into the nucleon properties and provides a precision benchmark for all nonperturbative QCD models.