  It has recently been shown that some integrable spin chains possess a set of
quasilocal conserved charges, with the classic example being the
spin-$\frac{1}{2}$ XXZ Heisenberg chain. These charges have been proven to be
essential for properly describing stationary states after a quantum quench, and
must be included in the generalized Gibbs ensemble (GGE). We find that similar
charges are also necessary for the GGE description of integrable quantum field
theories with non-diagonal scattering. A stationary state in a non-diagonal
scattering theory is completely specified by fixing the mode-ocuppation density
distributions of physical particles, as well auxiliary particles which carry no
energy or momentum. We show that the set of conserved charges with integer
Lorentz spin, related to the integrability of the model, are unable to fix the
distributions of these auxiliary particles, since these charges can only fix
kinematical properties of physical particles. The field theory analogs of
quasilocal lattice charges are therefore necessary. As a concrete example, we
find the complete set of charges needed in the sine-Gordon model, by using the
fact that this field theory is recovered as the continuum limit of a spatially
inhomogeneous version of the XXZ chain. The set of quasilocal charges of the
lattice theory are shown to become a set local charges with fractional spin in
the field theory.
