RELAXATION AND KINETICS IN SCALAR FIELD-THEORIES

A new approach to the dynamics of relaxation and kinetics of thermaliz ation in a scalar held theory is presented that incorporates the relev ant time scales through the resummation of hard thermal loops. An alte rnative derivation of the kinetic equations for the ''quasiparticle'' distribution functions is obtained that allows a clear understanding o f the different ''coarse-graining'' approximations usually involved in a kinetic description. This method leads to a systematic perturbative expansion to obtain the kinetic equations including hard thermal loop resummation and to an improvement including renormalization, off-shel l effects, and contributions that change chemical equilibrium on short time scales. As a by-product of these methods we establish the equiva lence between the relaxation time scale in the linearized equation of motion of the quasiparticles and the thermalization time scale of the quasiparticle distribution function in the ''relaxation time approxima tion'' including hard thermal loop effects. Hard thermal loop resummat ion dramatically modifies the scattering rate for long wavelength mode s as compared to the usual (semi)classical estimate. Relaxation and ki netics are studied both in the unbroken and broken symmetry phases of the theory. The broken symmetry phase also provides the setting to obt ain the contribution to the kinetic equations from processes that invo lve decay of a heavy scalar into light scalar particles in the medium.