ππ correlations in γ+ A reactions

Preliminary differential cross-sections of the reactions A(γ,π0π0) and A(γ,π0π + + π0π-) with A=1H, 12C, and natPb are presented. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π0π0 channel. The dependence is not observed in the π0π± channel. The in-medium observation in the π0π0 channel is consistent with an in-medium modification of the ππ interaction in the I=J=0 channel, changing width and pole position of a ππ resonant state. (orig.)