Analysis of γn→nπ+π− and γn→pπ−π0 reactions from threshold up to 1 GeV

The final results of a bubble chamber experiment on double pion photoproduction in deuterium, from threshold up to 1 GeV, are reported. The total cross-sections of reactions γn→nπ−π+, γn→pπ−π0 and γp→pπ+π− are calculated, by using the spectator model. The squared-mass distributions have been fitted, and the cross-sections of reactions γn→Δ−π+, γn→Δ+π−→pπ0π− and γp→Δ++π− have been obtained. The general features of these cross-sections are consistent with a large nonresonant Δ production, accounted for in a gauge-invariant OPE model.