Quantum field theory of X-cube fracton topological order and robust degeneracy from geometry

We propose a quantum field theory description of the X-cube model of fracton topological order. The field theory is not (and cannot be) a topological quantum field theory (TQFT) since, unlike the X-cube model, TQFTs are invariant (i.e., symmetric) under continuous space-time transformations. However, the theory is instead invariant under a certain subgroup of the conformal group. We describe how braiding statistics and ground-state degeneracy are reproduced by the field theory, and how the the X-cube Hamiltonian and field theory can be minimally coupled to matter fields. We also show that even on a manifold with trivial topology, spatial curvature can induce a ground-state degeneracy that is stable to arbitrary local perturbations! Our formalism may allow for the description of other fracton field theories, where the only necessary input is an equation of motion for a charge density.