Derivative Chameleons
  We consider generalized chameleon models where the conformal coupling between
matter and gravitational geometries is not only a function of the chameleon
field \phi, but also of its derivatives via higher order co-ordinate
invariants. Specifically we consider the first such non-trivial conformal
factor A(\phi,X), where X is the canonical kinetic term for \phi. The
associated phenomenology is investigated and we show that such theories have a
new generic mass-altering mechanism, potentially assisting the generation of a
sufficiently large chameleon mass in dense environments. The most general
effective potential is derived for such derivative chameleon setups and
explicit examples are given. Interestingly this points us to the existence of a
purely derivative chameleon protected by a shift symmetry for \phi. We also
discuss potential ghost-like instabilities associated with mass-lifting
mechanisms and find another, mass-lowering and instability-free, branch of
solutions. This suggests that, barring fine-tuning, stable derivative models
are in fact typically anti-chameleons that suppress the field's mass in dense
environments. Furthermore we investigate modifications to the thin-shell regime
and prove a no-go theorem for chameleon effects in non-conformal geometries of
the disformal type.
