  This work constructs a well-defined and operational form factor expansion in
a model having a massless spectrum of excitations. More precisely, the dynamic
two-point functions in the massless regime of the XXZ spin-1/2 chain are
expressed in terms of properly regularised series of multiple integrals. These
series are obtained by taking, in an appropriate way, the thermodynamic limit
of the finite volume form factor expansions. The series are structured in way
allowing one to identify directly the contributions to the correlator stemming
from the conformal-type excitations on the Fermi surface and those issuing from
the massive excitations (deep holes, particles and bound states). The obtained
form factor series opens up the possibility of a systematic and exact study of
asymptotic regimes of dynamical correlation functions in the massless regime of
the XXZ spin $1/2$ chain. Furthermore, the assumptions on the microscopic
structure of the model's Hilbert space that are necessary so as to write down
the series appear to be compatible with any model -- not necessarily integrable
-- belonging to the Luttinger liquid universality class. Thus, the present
analysis provides also the phenomenological structure of form factor expansions
in massless models belonging to this universality class.
