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Photoproduction of $\pi^{0}\pi^{0}$ and $\pi^{0}\pi^{\pm}$ pairs from nucleihas been measured over a wide mass range ($^2$H, $^{7}$Li, $^{12}$C, $^{40}$Ca,and $^{\rm nat}$Pb) for photon energies from threshold to 600 MeV. Theexperiments were performed at the MAMI accelerator in Mainz, using the Glasgowphoton tagging spectrometer and a 4$\pi$ electromagnetic calorimeter consistingof the Crystal Ball and TAPS detectors. A shift of the pion-pion invariant massspectra for heavy nuclei to small invariant masses has been observed for$\pi^0$ pairs but also for the mixed-charge pairs. The precise results allowfor the first time a model-independent analysis of the influence of pionfinal-state interactions. The corresponding effects are found to be large andmust be carefully considered in the search for possible in-medium modificationsof the $\sigma$-meson. Results from a transport model calculation reproduce theshape of the invariant-mass distributions for the mixed-charge pairs betterthan for the neutral pairs, but also for the latter differences between modelresults and experiment are not large, leaving not much room for$\sigma$-in-medium modification.

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