  We develop a theory of damping of low energy, collective excitations in a
quasi-2D, homogenous, dipolar Bose gas at zero temperature, via processes
whereby an excitation decays into two excitations with lower energy. We find
that owing to the nature of the low energy spectrum of a quasi-2D dipolar gas,
such processes cannot occur unless the momentum of the incoming quasi-particle
exceeds a critical value k_{crit}. We find that as the dipolar interaction
strength is increased, this critical value shifts to larger momenta. Our
predictions can be directly verified in current experiments on dipolar Bose
condensates using Bragg spectroscopy, and provide valuable insight into the
quantum many-body physics of dipolar gases.
