  We calculate the phase space factor for a two-body decay in which one of the
products is a tachyon. Two threshold conditions, a lower and an upper one, are
derived in terms of the masses of the particles and the speed of a preferred
frame. Implicit in the derivation is a consistently formulated quantum field
theory of tachyons in which spontaneous Lorentz symmetry breaking occurs. The
result is to be contrasted with a parallel calculation by Hughes and
Stephenson, which, however, implicitly adheres to strict Lorentz invariance of
the underlying quantum field theory and produces the conclusion that there is
no threshold for this process.
