  We introduce a microscopic Hamiltonian model of a two level system with
many-body interactions with an environment whose excitation dynamics is fully
solved within the Keldysh formalism. If a particle starts in one of the states
of the isolated system, the return probability oscillates with the Rabi
frequency $\omega_{0}$. For weak interactions with the environment
$1/\tau_{\mathrm{SE}}<2\omega_{0},$ we find a slower oscillation whose
amplitude decays with a decoherence rate $1/\tau_{\phi}=1/(2\tau_{\mathrm{SE}%
})$. However, beyond a finite critical interaction with the environment,
$1/\tau_{\mathrm{SE}}>2\omega_{0}$, the decoherence rate becomes
$1/\tau_{\phi}\propto(\omega_{0}^{2})\tau_{\mathrm{SE}}$. The oscillation
period diverges showing a \emph{quantum dynamical phase transition}to a Quantum
Zeno phase.
