$\pi\pi$ correlations in $\gamma + A$ reactions

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Preliminary differential cross-sections of the reactions $A(\gamma,\pi^{0}\pi^{0})$ and $A(\gamma,\pi^{0}\pi^ + + \pi^{0}\pi^-)$ with A = 1 H, 12 C, and nat Pb are presented. A significant nuclear-mass dependence of the $\pi\pi$ invariant-mass distribution is found in the $\pi^{0}\pi^{0}$ channel. The dependence is not observed in the $\pi^{0}\pi^{\pm}$ channel. The in-medium observation in the $\pi^{0}\pi^{0}$ channel is consistent with an in-medium modification of the $\pi\pi$ interaction in the I = J = 0 channel, changing width and pole position of a $\pi\pi$ resonant state.