New collective mode due to collisional coupling

Starting from a nonmarkovian conserving relaxation time approximation forcollisions we derive coupled dispersion relations for asymmetric nuclearmatter. The isovector and isoscalar modes are coupled due to asymmetric nuclearmeanfield acting on neutrons and protons differently. A further coupling isobserved by collisional correlations. The latter one leads to the appearance ofa new soft mode besides isoscalar and isovector modes in the system. We suggestthat this mode might be observable in asymmetric systems. This soft modeapproaches the isovector mode for high temperatures. At the same time theisovector mode remains finite and approaches a constant value at highertemperatures showing a transition from zero sound like damping to first sound.The damping of the new soft mode is first sound like at all temperatures.Comment: Phys. Rev. C su