Elsevier

Nuclear Physics A

Volume 552, Issue 4, 22 February 1993, Pages 451-468
Nuclear Physics A

Threshold behaviour of the π+π invariant mass in nuclei

https://doi.org/10.1016/0375-9474(93)90278-6Get rights and content

Abstract

We present π+π invariant-mass distributions between 300 and 380 MeV for the 2H, 4He, 16O, 208Pb+, π+π) 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, πNππ 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+, ) reaction are also presented for some of the examined nuclei. The experimental results are discussed within the framework of the two available models.

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