Two pion production from nuclei

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Double pion photoproduction from nuclei has been used to investigate the in-medium modification of meson-meson interactions with the TAPS photon spectrometer at MAMI-B. Recent developments are reported. Hadron-hadron interactions, and meson production and decays involving light nuclei are also studied in hadron induced experiments. In particular, the physics program of the new WASA-at-COSY facility includes double pion production from light nuclei. Here, the efforts focus on the ABC effect, the puzzling low-mass enhancement in the ππ invariant mass spectrum, first observed in inclusive measurements of two-pion production in nuclear collisions. (author)