  A new derivation of quantum stochastic differential equation for the
evolution operator in the low density limit is presented. We use the
distribution approach and derive a new algebra for quadratic master fields in
the low density limit by using the energy representation. We formulate the
stochastic golden rule in the low density limit case for a system coupling with
Bose field via quadratic interaction. In particular the vacuum expectation
value of the evolution operator is computed and its exponential decay is shown.
