  Divergences that occur in density matrices of decay and scattering processes
are shown to be regularized by tracing and unitarity or the optical theorem.
These divergences are regularized by the lifetime of the decaying particle or
the total scattering cross section. Also, this regularization is shown to give
the expected helicities of final particles. The density matrix is derived for
the weak decay of a polarized muon at rest, $\mu^- \rightarrow \nu_{\mu} (e^-
\bar \nu_e)$, with Lorentz invariant density matrix entries and unitarity
upheld at tree level. The electron's von Neumann entanglement entropy
distributions are calculated with respect to both the electron's emission angle
and energy. The angular entropy distribution favors an electron emitted
backwards with respect to the muon's polarization given a minimum volume
regularization. The kinematic entropy distribution is maximal at half the
muon's rest mass energy. These results are similar to the electron's angular
and kinematic decay rate distributions. Both the density matrix and
entanglement entropy can be cast either in terms of ratios of areas or volumes.
