In-medium modifications of the $\ddot{I}€\ddot{I}€$ interaction in photon-induced reactions

Abstract

Differential cross sections of the reactions $({\hat{I}}^3,\ddot{I}{€}^{\circ }\ddot{I}{€}^{\circ })$ and $({\hat{I}}^3,\ddot{I}{€}^{\circ }\ddot{I}{€}^{+}+\ddot{I}{€}^{\circ }\ddot{I}{€}^{-})$ have been measured for several nuclei (${}^1$H,${}^{12}$C, and ${}^{\mathrm{nat}}$Pb) at an incident-photon energy of $E_{\hat{I}}^3$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $\ddot{I}€\ddot{I}€$ invariant-mass distribution is found in the $\ddot{I}{€}^{\circ }\ddot{I}{€}^{\circ }$ channel. This dependence is not observed in the $\ddot{I}{€}^{\circ }\ddot{I}{€}^{+/-}$ channel and is consistent with an in-medium modification of the $\ddot{I}€\ddot{I}€$ interaction in the $I$=$J$=0 channel. The data are compared to $\ddot{I}€$-induced measurements and to calculations within a chiral-unitary approach.