Abstract
Beam-helicity asymmetries and mass-differential cross sections have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral and mixed-charge pion pairs in the reactions γp → nπ0π+ and γp → pπ0π0 off free protons and γd → (p)pπ0π-, γd → (n)pπ0π0 and γd → (n)nπ0π+, γd → (p)nπ0π0 off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced in bremsstrahlung processes of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The decay products (photons, protons neutrons and charged pions) were detected in the 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a full kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, indicate that the quasi-free neutron results are also a close approximation of the free-neutron results. Comparison of the results to predictions of model calculations portend that especially the reaction mechanisms in the production of the mixed-charge final states are still not well understood, in particular at low incident photon energies in the second nucleon resonance region.
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