Behind Horndeski: Structurally Robust Higher Derivative EFTs

TL;DR

This paper classifies higher derivative scalar interactions that are free of ghosts and stable under quantum corrections, providing a robust effective field theory framework for Horndeski and beyond Horndeski theories with applications to cosmology.

Contribution

It offers a complete classification of ghost-free higher derivative operators with a power counting scheme based on approximate global symmetries, enhancing the theoretical robustness of Horndeski-like models.

Findings

Identifies a set of higher derivative operators free of ghosts.

Provides a power counting rule for coefficients based on symmetries.

Connects the classification to Horndeski and beyond Horndeski theories.

Abstract

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.