ππ pairs in nuclei and the σ meson

Abstract

In-medium pion-pion correlations were studied via their effect on experimental observables measured for the ${\ensuremath{\pi}}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ reaction in nuclei $(A:{ }^{12}{\mathrm{C},\mathrm{ }}^{40}\mathrm{Ca}$, and ${}^{208}\mathrm{Pb})$ 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 $\ensuremath{\pi}\ensuremath{\pi}$ invariant mass distributions ${M}_{\ensuremath{\pi}\ensuremath{\pi}}$ were measured from the ${2m}_{\ensuremath{\pi}}$ threshold up to the kinematic maximum, and total cross sections ${\ensuremath{\sigma}}_{T}$ deduced. The $\ensuremath{\pi}\ensuremath{\pi}$ medium modifications were investigated by forming the composite ratio ${C}_{\ensuremath{\pi}\ensuremath{\pi}}^{A}{=(M}_{\ensuremath{\pi}\ensuremath{\pi}}^{A}/{\ensuremath{\sigma}}_{T}^{A}{)/(M}_{\ensuremath{\pi}\ensuremath{\pi}}^{N}/{\ensuremath{\sigma}}_{T}^{N})$ which proved to be only weakly affected by the $(\ensuremath{\pi},2\ensuremath{\pi})$ reaction mechanism. In the $I=J=0$ channel the ${C}_{\ensuremath{\pi}\ensuremath{\pi}}^{A}$ distributions are peaked at the ${2m}_{\ensuremath{\pi}}$ threshold and increases with A. For pion pairs in the $I=2,$ $J=0$ channel the ${C}_{\ensuremath{\pi}\ensuremath{\pi}}^{A}$ distributions are nearly independent of A. The distinctive behavior of ${C}_{\ensuremath{\pi}\ensuremath{\pi}}^{A}$ may be related to the appearance of the \ensuremath{\sigma} meson.