A dependence of the (pi+,pi+ pi+-) reaction near the 2m(pi) threshold

The 2H,12C,40Ca,208Pb(\u3c0+,\u3c0+\u3c0\ub1) reactions have been studied at T\u3c0+ = 282.7 MeV, and the \u3c0+\u3c0\ub1 invariant mass distributions M\u3c0+\u3c0\ub1A measured down to the 2m\u3c0 threshold. The M\u3c0+\u3c0-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 \u3c0+\u3c0- pairs in this range of M\u3c0+\u3c0-A have J = I = 0. In the case of M\u3c0+\u3c0+A, there is some strength near threshold regardless of A, and the \u3c0+\u3c0+ pairs have J = 0,I = 2. The experimental results indicate that nuclear matter strongly modifies the \u3c0\u3c0 interaction in the J = I = 0 channel