  Using non-relativistic 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 on deriving hydrodynamic
equations from quantum theory.
