A Hidden Symmetry of the Galileon

TL;DR

The paper reveals a special parameter choice in galileon theories that introduces an enhanced quadratic shift symmetry, explaining improved soft limit behavior and constraining the theory's form.

Contribution

It identifies a unique symmetry in galileon theories that constrains their structure and explains their soft limit properties.

Findings

Enhanced shift symmetry exists for specific parameters.

This symmetry constrains the theory to even powers of the field.

It accounts for improved soft limit behavior in the quartic galileon S-matrix.

Abstract

We show that there is a special choice of parameters for which the galileon theory is invariant under an enhanced shift symmetry whose non-linear part is quadratic in the coordinates. This symmetry fixes the theory to be equivalent to one with only even powers of the field, with no free coefficients, and accounts for the improved soft limit behavior observed in the quartic galileon S-matrix.