The ππ interaction in nuclear matter from a study of the π+A→π+π±A′ reactions Original Research Article

The pion-production reactions π+A→π+π±A′ were studied on 2H, 12C, 40Ca, and 208Pb nuclei at an incident pion energy of View the MathML source MeV. Pions were detected in coincidence using the CHAOS spectrometer. The experimental results are reduced to differential cross sections and compared to both theoretical predictions and the reaction phase space. The composite ratio View the MathML sourceView the MathML source between the π+π± invariant masses on nuclei and on the nucleon is also presented. Near the View the MathML source threshold pion pairs couple to View the MathML source when produced in the π+→π+π− reaction channel. There is a marked near-threshold enhancement of View the MathML source which is consistent with theoretical predictions addressing the partial restoration of chiral symmetry in nuclear matter. Furthermore, the behaviour of View the MathML source is well described when the restoration of chiral symmetry is combined with standard P-wave renormalization of pions in nuclear matter. On the other hand, nuclear matter only weakly influences View the MathML source, which displays a flat behaviour throughout the energy range regardless of A.