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$.
Categories: nucl-ex