2π0 photoproduction from nuclei

The photoproduction of π pairs (π0π0, π0π+) from 40Ca has been measured with high statistical accuracy with the TAPS detector at the Mainz MAMI accelerator. Total cross sections and invariant mass distributions of the ππ pairs have been measured for incident photon energies from threshold up to 820 MeV. Evidence of a nuclear-mass dependence of the ππ invariant-mass distribution is found in the π0π0 channel while this dependence is not observed in the π0π± channel. This indicates an in-medium modification of the ππ interaction in the I=J=0 channel. The total cross sections for both double pion channels agree with the elementary cross sections on the free nucleon, when the Ca cross section is scaled by A2/3