Thermal Behaviour of $\pi-\pi$ Scattering Lengths in the   Nambu--Jona-Lasinio model
  We calculate the thermal evolution of $\pi-\pi$ scattering lengths, in the frame of the Nambu--Jona-Lasinio model. The thermal corrections were calculated at the one loop level using Thermofield Dynamics. We present also results for the pion thermal mass. Our procedure implies the modeling of a propagating scalar meson as a resumation of chains of quark bubbles, which is presented explicitly. We compare our results with previous analysis of this problem in the frame of different theoretical approaches. 
M. Loewe, Jorge Ruiz A. and J. C. Rojas