Two pion photoproduction on nucleons and nuclei in the rho and sigma regions

In this talk we report on two physical processes, the photoproduction of $\rho$ mesons in nuclei and the $\pi^0 \pi^0$ photoproduction in nuclei at low energies. In the first case the aim is to observe experimentally the theoretically predicted changes on the $\rho $ properties in the nuclear medium. In the second case one wishes to investigate the modifications of the $\pi \pi$ interaction in the nuclear medium in the region of the $\sigma$ meson. In the first case we observe that it is quite difficult to see large effects due to the $\rho$ modification in the medium because the detection of the $\rho $ is done through the two pions which are produced in the $\Delta$ region and are largely distorted in the medium. In the second case, when the I=0 part of the $\pi \pi$ interaction is substituted by the in medium $\pi \pi$ amplitude, we observe a very large shift of strength in the invariant mass distribution to small values of the mass in $^{12}C$ and $^{208}Pb$ with respect to the distribution of the elementary reaction. This spectacular shift appears to be corroborated by recent experiments at Mainz reported in this same Workshop.