  Two-component BECs subject to laser-induced coupling exhibit a complicated
spectrum of excitations, which can be viewed as two interacting phonon modes.
We study the conditions required to make these two phonon modes decouple. Once
decoupled, the phonons not only can be arranged travel at different speeds, but
one of the modes can be given a mass -- it exhibits the dispersion relation of
a massive relativistic particle: omega = sqrt{omega_0^2 + c^2 k^2}.
  This is a new and unexpected excitation mode for the coupled BEC system.
Apart from its intrinsic interest to the BEC community, this observation is
also of interest for the ``analogue gravity'' programme, as it opens the
possibility for using BECs to simulate massive relativistic particles in an
effective ``acoustic geometry''.
