  According to the Bogoliubov theory the low energy behaviour of the Bose gas
at zero temperature can be described by non-interacting bosonic quasiparticles
called phonons. In this work the damping rate of phonons at low momenta, the
so-called Beliaev damping, is explained and computed with simple arguments
involving the Fermi Golden Rule and Bogoliubov's quasiparticles.
