  The unconventional low-lying spin excitations, recently observed in neutron
scattering experiments on ${\rm Cs_2 Cu Cl_4}$, are explained with a spin
liquid wave function. The dispersion relation as well as the wave vector of the
incommensurate spin correlations are well reproduced within a projected BCS
wave function with gapless and fractionalized spin-1/2 excitations around the
nodes of the BCS gap function. The proposed wave function is shown to be very
accurate for one-dimensional spin-1/2 systems, and remains similarly accurate
in the two-dimensional model corresponding to ${\rm Cs_2 Cu Cl_4}$, thus
representing a good ansatz for describing spin fractionalization in two
dimensions.
