pi-pi correlations in photon-induced reactions

Differential cross sections of the-reactions A(gamma,pi(0) pi(0)) and A(gamma, pi(0) pi(+) + pi(0) pi(-)) with A=H-1, C-12, and Pb-nat are presented. A significant. nuclear-mass dependence of the pipi invariant-mass distribution is found in the pi(0) pi(0) channel. The dependence is not observed in the pi(0) pi(+/-) channel. Within a unitary chiral-perturbation approach,(5)) the in-medium observation in the pi(0) pi(0) channel is interpreted as an in-medium modification of the pipi interaction in the sigma (I=J=0) channel, changing width and pole position with increasing effective nuclear density.