  We investigate the different photon emission regimes created by a preexcited
and collimated atomic beam passing through a single mode of an optical cavity.
In the regime where the cavity degrees of freedom can be adiabatically
eliminated, we find that the atoms undergo superradiant emission when the
collective linewidth exceeds the transit-time broadening. We analyze the case
where the atomic beam direction is slanted with respect to the cavity axis. For
this situation, we find that a phase of continuous light emission similar to
steady-state superradiance is established providing the tilt of the atomic beam
is sufficiently small. However, if the atoms travel more than half a wavelength
along the cavity axis during one transit time we predict a dynamical phase
transition to a new bistable superradiant regime. In this phase the atoms
undergo collective spontaneous emission with a frequency that can be either
blue or red detuned from the free-space atomic resonance. We analyze the
different superradiant regimes and the quantum critical crossover boundaries.
In particular we find the spectrum of the emitted light and show that the
linewidth exhibits features of a critical scaling close to the phase
boundaries.
