STATISTICAL QUASI-PARTICLES IN TRANSVERSE DYNAMICS OF GASES

We analyze the validity of the Fermi-liquid approach to transverse dyn amics of spin-polarized gases at arbitrary temperatures. We demonstrat e that the diagrammatic kinetic equation for transverse processes can be formulated as a simpler, but completely equivalent equation in term s of ''statistical quasiparticles.'' The equation includes all coheren t and dephasing molecular-field terms as well as the dissipative colli sion integral up to the second order. Beyond the second order, the res ults become very complicated, and a quasiparticle approach loses its a ttraction. We give the expressions for the effective interaction funct ion and collision integral for statistical quasiparticles, applicable at all temperatures, and discuss the implications of this concept at h igh temperatures. The interaction function contains anomalous pole ter ms which do not exist in equations for longitudinal dynamics. This pro vides a somewhat unexpected interpretation for zero-temperature dissip ative processes, observed recently in spin dynamics, and for controver sial molecular-held terms (the so-called It terms) as imaginary (pole) and real (principal) parts of the quasiparticle interaction function. These molecular-held terms with complicated analytical structure do n ot vanish completely, as was assumed earlier, in the Boltzmann region, but contribute to higher-order density terms. With an emphasis on qua ntum gases, we discuss how to reconcile various physical assumptions i nherent to different kinetic approaches to dilute gases.