Abstract
We analyze a theory known as extended DBI, which interpolates between DBI and the U(N) x U(N)/U(N) non-linear sigma model and represents a nontrivial example of theories with mixed power counting. We discuss symmetries of the action and their geometrical origin; the special case of SU(2) extended DBI theory is treated in great detail.
The revealed symmetries lead to a new type of graded soft theorem that allows us to prove on-shell constructibility of the tree-level S-matrix. It turns out that the on-shell constructibility of the full extended DBI remains valid, even if its DBI sub-theory is modified in such a way to preserve its own on-shell constructibility.
We thus propose a slight generalization of the DBI sub-theory, which we call 2-scale DBI theory. Gluing it back to the rest of the extended DBI theory gives a new set of on-shell reconstructible theories - the 2-scale extended DBI theory and its descendants.
The uniqueness of the parent theory is confirmed by the bottom-up approach that uses on-shell amplitude methods exclusively.