  The challenge of understanding the dynamics of a mobile impurity in an
interacting quantum many-body medium comes from the necessity of including
entanglement between the impurity and excited states of the environment in a
wide range of energy scales. In this paper, we investigate the motion of a
finite mass impurity injected into a three-dimensional quantum Bose fluid as it
starts shedding Bogoliubov excitations. We uncover a transition in the dynamics
as the impurity's velocity crosses a critical value which depends on the
strength of the interaction between the impurity and bosons as well as the
impurity's recoil energy. We find that in injection experiments, the two
regimes differ not only in the character of the impurity velocity abatement,
but also exhibit qualitative differences in the Loschmidt echo, density ripples
excited in the BEC, and momentum distribution of scattered bosonic particles.
The transition is a manifestation of a dynamical quantum Cherenkov effect, and
should be experimentally observable with ultracold atoms using Ramsey
interferometry, RF spectroscopy, absorption imaging, and time-of-flight
imaging.
