Interacting pion pairs in nuclear matter

The pion-production $\pi^+ A \to \pi^+\pi^{\pm} A'$ reactions were studied on nuclei $^{2}H$, $^{12}C$, $^{40}Ca$ and $^{208}Pb$ at a pion energy of $T_{\pi^{+}}$=283 MeV using the CHAOS spectrometer. The experimental results are reduced to differential cross sections and compared to both theoretical predictions and reaction phase space. Near the $2m_{\pi}$ threshold pion pairs couple to $(\pi\pi)_{I=J=0}$ when produced in the $\pi^+\to \pi^+\pi^-$ reaction channel. The $\cal C$$_{\pi\pi}^A$ ratio between the $\pi^{+}\pi^{\pm}$ invariant masses on nuclei and on the nucleon is also presented. The marked near-threshold enhancement of $\cal C$$_{\pi^+\pi^-}^A$ is consistent with theoretical predictions addressing the partial restoration of chiral symmetry. On the opposite, nuclear matter weakly influences $\cal C$$_{\pi^+\pi^+}^A$.