ππ pairs in nuclei and the σ meson -: art. no. 018201

In-medium pion-pion correlations were studied via their effect on experimental observables measured for the pi(+)-->pi(+)pi(+/1) reaction in nuclei (A: C-12, Ca-40, and Pb-208) and in deuterium, from which it is inferred the observables for the nucleon (N: rz and p). The measurements were performed at TRIUMF using a positive-pion flux with a bombarding energy of 283 MeV. The pi pi invariant mass distributions M-pi pi were measured from the 2m(pi) threshold up to the kinematic maximum, and total cross sections sigma(T) deduced. The pi pi medium modifications were investigated by forming the composite ratio C-pi pi(A) = (M-pi pi(A)/sigma(T)(A))/(M-pi pi(N)/sigma(T)(N)) which proved to be only weakly affected by the (pi,2 pi) reaction mechanism. In the I = J = 0 channel the C-pi pi(A) distributions are peaked at the 2m(pi) threshold and increases with A. For pion pairs in the I = 2, J = 0 channel the C-pi pi(A) distributions are nearly independent of A. The distinctive behavior of Ca, may be related to the appearance of the sigma meson. [S0556-2813(99)00707-4].