Behind Horndeski: Structurally Robust Higher Derivative EFTs
  Higher derivative scalar interactions can give rise to interesting
cosmological scenarios. We present a complete classification of such operators
that can yield sizeable effects without introducing ghosts and, at the same
time, define an effective field theory robust under the inclusion of quantum
corrections. A set of rules to power count consistently the coefficients of the
resulting Lagrangian is provided by the presence of an approximate global
symmetry. The interactions that we derive in this way contain a subset of the
so-called Horndeski and beyond Horndeski theories. Our construction therefore
provides a structurally robust context to study their phenomenology.
Applications to dark energy/modified gravity and geodesically complete
cosmologies are briefly discussed.
