The scalar-isoscalar spectral function of strong matter in a large N   approximation
  The enhancement of the scalar-isoscalar spectral function near the two-pion threshold is studied in the framework of an effective linear $\sigma$ model, using a large N approximation in the number of the Goldstone bosons. The effect is rather insensitive to the detailed T=0 characteristics of the $\sigma$ pole, it is accounted by a pole moving with increasing $T$ along the real axis of the second Riemann sheet towards the threshold location from below. 
A. Patkos, Zs. Szep, P. Szepfalusy