Evaluation of the E2/M1 Ratio in the N→Δ(1232) Transition from the Γ⃗p⃗→ P Π^0 Reaction

A new data set for the helicity-dependent differential cross section of thesingle-meson photoproduction reaction γ p → p π^0 was obtained forthe photon energy interval 150-400 MeV. The experiment was performed at the A2tagged photon facility of the Mainz Microtron MAMI using a circularly polarizedphoton beam and a longitudinally polarized proton target. The reaction productswere detected with the large acceptance Crystal Ball/TAPS calorimeter covering97% of the full solid angle. These new results, obtained with a fine energyand polar angle binning, greatly increase both the existing quantity andquality of the data available for this observable. A moment analysis, based ona finite expansion in Legendre polynomials, was applied to these data by usinga bootstrap-based fitting method to correctly account for their systematicuncertainties. From the resulting decomposition of the differential crosssections, the E2/M1 ratio for the N→Δ(1232) transition wasdetermined to be [-2.38 ± 0.16 (stat.+sys.)± 0.10 (model)] %. Combining this value with previous results also allowed us toevaluate the most precise available estimate of the E2/M1 ratio to be usedfor all further reference and model comparisons.