In-medium modifications of the $\pi \pi$ interaction in photon-induced reactions

Abstract

Differential cross sections of the reactions $(\gamma ,{\pi }^{\circ }{\pi }^{\circ })$ and $(\gamma ,{\pi }^{\circ }{\pi }^{+}+{\pi }^{\circ }{\pi }^{-})$ have been measured for several nuclei (${}^1$H,${}^{12}$C, and ${}^{\mathrm{nat}}$Pb) at an incident-photon energy of $E_{\gamma }$=400-460 MeV 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 ${\pi }^{\circ }{\pi }^{\circ }$ channel. This dependence is not observed in the ${\pi }^{\circ }{\pi }^{+/-}$ 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.

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