Evaluation of the E2/M1 Ratio in the N → Δ(1232) Transition from the (Γ)over-Right-arrow(p)over-right-arrow → Pπ0 Reaction

A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction gamma p -> p pi(0) was obtained for the photon energy interval 150-400 MeV. The experiment was performed at the A2 tagged photon facility of the Mainz Microtron MAMI using a circularly polarized photon beam and a longitudinally polarized proton target. The reaction products were detected with the large-acceptance Crystal Ball and TAPS calorimeters covering 97% of the full solid angle. These new results, obtained with a fine energy and polar angle binning, greatly increase both the existing quantity and quality of the data available for this observable. A moment analysis, based on a finite expansion in Legendre polynomials, was applied to these data by using a bootstrap-based fitting method to correctly account for their systematic uncertainties. From the resulting decomposition of the differential cross sections, the E2/M1 ratio for the N -> Delta(1232) transition was determined to be [-2.38 +/- 0.16(stat.+sys.) +/- 0.10 (model)]%. Combining this value with previous results also allowed us to evaluate the most precise available estimate of the E2/M1 ratio to be used for all further reference and model comparisons.