Pion pairs in nuclear matter

Abstract

The pion-production reaction π+A→π+π±A′ as a function of A and T has been studied with the CHAOS spectrometer at the TRIUMF meson facility. Charged pion pairs have been detected in coincidence to ensure a unique identification of π+π± events. The π+π− invariant mass Mπ+π−A has been found to peak increasingly toward the 2mπ threshold as the nucleus mass number increases, while the Mπ+π+A yield shows a negligible A-dependence near threshold. The π+π− pairs have been found to predominantly couple to S-wave, i.e., J=0. Theoretical works on the (ππ)I=J=0 interaction in nuclear matter, have shown that such enhancement is the combined effect of standard many-body correlations and the restoration of chiral symmetry in nuclear matter, i.e., the appearance of the σ-meson. New data for the pion-production reaction on 45Sc at three incident kinetic energies (240, 280, 300 MeV) have been analyzed and preliminary results are presented. The results for 45Sc at 280 MeV are compared with the 40Ca results, previously published by the CHAOS collaboration. The 45Sc results at different energies highlight the T-dependence of the ππ interaction in nuclear matter.