Quasi-free photoproduction of pion-pairs off protons and neutrons
  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 $\boldsymbol{\gamma}p\rightarrow n\pi^0\pi^+$ and $\boldsymbol{\gamma}p\rightarrow p\pi^0\pi^0$ off free protons and $\boldsymbol{\gamma}d \rightarrow (p)p\pi^0\pi^-$, $\boldsymbol{\gamma}d\rightarrow (n)p\pi^0\pi^0$ and $\boldsymbol{\gamma}d\rightarrow (n)n\pi^0\pi^+$, $\boldsymbol{\gamma}d\rightarrow (p)n \pi^0\pi^0$ 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 $4\pi$ 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, indicating that the quasi-free neutron results are also a close approximation of the free-neutron results. Comparisons of the results to predictions from model calculations suggest 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. 
M. Oberle and B. Krusche
physics.acc-ph hep-ex nucl-ex