Within the generalized hydrodynamics (GHD) formalism for quantum
integrable models, it is possible to compute simple expressions for a
number of correlation functions at the Eulerian scale. Specializing to
integrable relativistic field theories, we show the same correlators can
be computed as a sum over form factors, the GHD regime corresponding to
the leading contribution with one particle-hole pair on a finite
energy-density background. The thermodynamic bootstrap program (TBP)
formalism was recently introduced as an axiomatic approach to computing
such finite-energy-density form factors for integrable field theories.
We derive a new axiom within the TBP formalism from which we easily
recover the predicted GHD Eulerian correlators. We also compute higher
form factor contributions, with more particle-hole pairs, within the
TBP, allowing for the computation of correlation functions in the
diffusive, and beyond, GHD regimes. The two particle-hole form factors
agree with expressions recently conjectured within the GHD.
An optical-lattice-based quantum simulator for relativistic field theories and topological insulators