Decoherence and records for the case of a scattering environment

Using nonrelativistic many body quantum field theory, a master equation is derived for the reduced density matrix of a dilute gas of massive particles undergoing scattering interactions with an environment of light particles. The dynamical variable that naturally decoheres (the pointer basis) is essentially the local number density of the dilute gas. Earlier master equations for this sort of system (such as that derived by Joos and Zeh) are recovered on restricting to the one-particle sector for the distinguished system. The derivation shows explicitly that the scattering environment stores information about the system by "measuring" the number density. This therefore provides an important example of the general connection between decoherence and records indicated by the decoherent histories approach to quantum theory. It also brings the master equation for this system into a form emphasizing the role of local densities, which is relevant to current work in deriving hydrodynamic equations from quantum theory.