Threshold behavior of the pi+ pi- invariant mass in nuclei

We present π+π− invariant-mass distributions between 300 and 380 MeV for the 2H, 4He, 16O, View the MathML source reaction at an incident π+ energy of 280 MeV. A prominent feature is the shift of their maximum downward to the 2mπ 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 (ππ)I = L = 0 system embedded in the nuclear medium. Also, the π+π− invariant-mass cross sections are compared with the predictions of an A(π+, π+π−) model that uses a microscopical approach for the elementary process of pion production, πN → ππ N. Four-fold differential cross sections as a function of the energy of outgoing π+ and p, and missing-mass distributions for the A(π+, pπ−) reaction are also presented for some of the examined nuclei. The experimental results are discussed within the framework of the two available models.