Double-Pion Production in γ+A Reactions

Preliminary differential cross sections of the reactions A(γ, π° π°) and A(γ, π° π°⁺ + π°π^-) with A=¹H, ¹²C, and ^natPb are presented. A significant nuclear-mass dependence of the ππ invariant-mass distribution is found in the π°π° channel. The dependence is not observed in the π°π^± channel. It 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.