  We propose a form factor approach for the computation of the large distance
asymptotic behavior of correlation functions in quantum critical (integrable)
models. In the large distance regime we reduce the summation over all excited
states to one over the particle/hole excitations lying on the Fermi surface in
the thermodynamic limit. We compute these sums, over the so-called critical
form factors, exactly. Thus we obtain the leading large distance behavior of
each oscillating harmonic of the correlation function asymptotic expansion,
including the corresponding amplitudes. Our method is applicable to a wide
variety of integrable models and yields precisely the results stemming from the
Luttinger liquid approach, the conformal field theory predictions and our
previous analysis of the correlation functions from their multiple integral
representations. We argue that our scheme applies to a general class of
non-integrable quantum critical models as well.
