In-medium modifications of the $ππ$ interaction in photon-induced reactions
Differential cross sections of the reactions $(γ,π^\circπ^\circ)$ and $(γ,π^\circπ^++π^\circπ^-)$ have been measured for several nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of $E_γ$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $ππ$ invariant-mass distribution is found in the $π^\circπ^\circ$ channel. This dependence is not observed in the $π^\circπ^{+/-}$ channel and is consistent with an in-medium modification of the $ππ$ interaction in the $I$=$J$=0 channel. The data are compared to $π$-induced measurements and to calculations within a chiral-unitary approach.
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