THRESHOLD BEHAVIOR OF THE PI+PI- INVARIANT MASS IN NUCLEI

We present pi+pi- invariant-mass distributions between 300 and 380 MeV for the H-2, He-4, O-16, Pb-208(pi+, pi+pi-) reaction at an incident pi+ energy of 280 MeV. A prominent feature is the shift of their maximum downward to the 2m(pi) threshold as A increases. This behaviour finds a qualitative explanation in the framework of a model that investigates the modification of the strength function for an interacting (pipi)I=L=0 system embedded in the nuclear medium. Also, the pi+pi- invariant-mass cross sections are compared with the predictions of an A(pi+, pi+pi-) model that uses a microscopical approach for the elementary process of pion production, piN --> pipiN. Four-fold differential cross sections as a function of the energy of outgoing pi+ and p, and missing-mass distributions for the A(pi+, ppi-) reaction are also presented for some of the examined nuclei. The experimental results are discussed within the framework of the two available models.