Double pion photoproduction off nuclei - Are there effects beyond final-state interaction?

Photoproduction of pi(0)pi(0) and pi(0)pi(+/-) pairs from nuclei has been measured over a wide mass range (H-2, Li-7, C-12, Ca-40, and Pb-nat) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4 pi electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant-mass spectra for heavy nuclei to small invariant masses has been observed for pi(0) pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the sigma-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for sigma-in-medium modification. (C) 2013 Elsevier B.V. All rights reserved