  We use quantum field theory methods to study the network model for the spin
quantum hall transition. When the couplings are fine tuned in a certain way,
the spin and charge degrees of freedom, corresponding to the supercurrent
algebras su(2) and osp(2|2) respectively, decouple in the renormalization group
flow. The infrared fixed point of this simpler theory is the coset
osp(4|4)/su(2) which is closely related to the current algebra osp(2|2) but not
identical. Some critical exponents are computed and shown to agree with the
recent predictions based on percolation.
