Evaluation of the E2/M1 ratio in the N→ Δ(1232) transition from the γ- p- →pπ0 reaction

A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction γp→pπ0 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→Δ(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.