  The ground state properties and neutron structure factor for the
two-dimensional antiferromagnet on the triangular lattice, with uni-directional
anisotropy in the nearest-neighbor exchange couplings and a weak
Dzyaloshinskii-Moriya (DM) interaction, are studied. This Hamiltonian has been
used to interpret neutron scattering measurements on the spin 1/2 spiral
spin-density-wave system, Cs_2CuCl_4, [R. Coldea, et al., Phys. Rev. B 68,
134424 (2003)]. Calculations are performed using a 1/S expansion, taking into
account interactions between spin-waves. The ground state energy, the shift of
the ordering wave-vector, Q, and the local magnetization are all calculated to
order 1/S^2. The neutron structure factor, obtained using anharmonic spin-wave
Green's functions to order 1/S, is shown to be in reasonable agreement with
published neutron data, provided that slightly different parameters are used
for the exchange and DM interactions than those inferred from measurements in
high magnetic field.
