The ${}^2\mathrm{H}{,}^{12}\mathrm{C}{,}^{40}\mathrm{Ca}{,}^{208}\mathrm{Pb}({\pi }^{+},{\pi }^{+}{\pi }^{\pm })$ reactions have been studied at $T_{{\pi }^{+}}=282.7\mathrm{MeV}$, and the ${\pi }^{+}{\pi }^{\pm }$ invariant mass distributions $M_{{\pi }^{+}{\pi }^{\pm }}^A$ measured down to the ${2m}_{\pi }$ threshold. The $M_{{\pi }^{+}{\pi }^{-}}^A$ yield near threshold is close to zero for $A=2,$ and increases dramatically with increasing A (from 12 to 40 to 208). A phase analysis indicates that the ${\pi }^{+}{\pi }^{-}$ pairs in this range of $M_{{\pi }^{+}{\pi }^{-}}^A$ have $J=I=0\mathrm{}$. In the case of $M_{{\pi }^{+}{\pi }^{+}}^A,$ there is some strength near threshold regardless of A, and the ${\pi }^{+}{\pi }^{+}$ pairs have $J=0,I=2\mathrm{}$. The experimental results indicate that nuclear matter strongly modifies the ππ interaction in the $J=I=0\mathrm{}$ channel.

• Received 14 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.603