Supersymmetric Cubic Galileons Have Ghosts
  Galileons are higher-derivative theories of a real scalar which nevertheless
admit second order equations of motion. They have interesting applications as
dark energy models and in early universe cosmology, and have been conjectured
to arise as descriptions of brane dynamics in string theory. In the present
paper, we study the bosonic sector of globally N=1 supersymmetric extensions of
the cubic Galileon Lagrangian in detail. Supersymmetry requires that the
Galileon scalar now becomes paired with a second real scalar field. We prove
that the presence of this second scalar causes the equations of motion to
become higher than second order, thus leading to the appearance of ghosts. We
also analyze the energy scales up to which, in an effective field theory
description, the ghosts can be tamed.
