ππ pairs in nuclei and the σ meson

In-medium pion-pion correlations were studied via their effect on experimental observables measured for the π⁺→π⁺π^± reaction in nuclei (A: ¹²C, ⁴⁰Ca, and ²⁰⁸Pb) and in deuterium, from which it is inferred the observables for the nucleon (N: n and p). The measurements were performed at TRIUMF using a positive-pion flux with a bombarding energy of 283 MeV. The ππ invariant mass distributions M_ππ were measured from the 2m_π threshold up to the kinematic maximum, and total cross sections σ_T deduced. The ππ medium modifications were investigated by forming the composite ratio C_ππ^A=(M_ππ^A/σ_T^A)/(M_ππ^N/σ_T^N) which proved to be only weakly affected by the (π,2π) reaction mechanism. In the I=J=0 channel the C_ππ^A distributions are peaked at the 2m_π threshold and increases with A. For pion pairs in the I=2, J=0 channel the C_ππ^A distributions are nearly independent of A. The distinctive behavior of C_ππ^A may be related to the appearance of the σ meson.