Differential cross sections of the reactions $(\gamma ,{\pi }^0{\pi }^0)$ and $(\gamma ,{\pi }^0{\pi }^{+/-})$ have been measured for several nuclei (${}^{\mathrm{1}}\mathrm{H}$,${}^{\mathrm{12}}\mathrm{C}$, and ${}^{\mathrm{n}\mathrm{a}\mathrm{t}}\mathrm{P}\mathrm{b}$) at an incident-photon energy of $E_{\gamma }=400–460\mathrm{M}\mathrm{e}\mathrm{V}$ at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $\pi \pi$ invariant-mass distribution is found in the ${\pi }^0{\pi }^0$ channel. This dependence is not observed in the ${\pi }^0{\pi }^{+/-}$ channel and is consistent with an in-medium modification of the $\pi \pi$ interaction in the $I=J=0$ channel. The data are compared to $\pi$-induced measurements and to calculations within a chiral-unitary approach.

• Received 15 May 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.222302