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

We present \u3c0+\u3c0 12 invariant-mass distributions between 300 and 380 MeV for the 2H, 4He, 16O, View the MathML source reaction at an incident \u3c0+ energy of 280 MeV. A prominent feature is the shift of their maximum downward to the 2m\u3c0 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 (\u3c0\u3c0)I = L = 0 system embedded in the nuclear medium. Also, the \u3c0+\u3c0 12 invariant-mass cross sections are compared with the predictions of an A(\u3c0+, \u3c0+\u3c0 12) model that uses a microscopical approach for the elementary process of pion production, \u3c0N \u2192 \u3c0\u3c0 N. Four-fold differential cross sections as a function of the energy of outgoing \u3c0+ and p, and missing-mass distributions for the A(\u3c0+, p\u3c0 12) reaction are also presented for some of the examined nuclei. The experimental results are discussed within the framework of the two available models