  We propose a critical spin liquid ground state for S=1/2 antiferromagnets on
the square lattice. In a renormalization group analysis of the `staggered flux'
algebraic spin liquid, we examine perturbations, present in the
antiferromagnet, which break its global SU(4) symmetry to SO(5). At physical
parameter values, we find an instability towards a fixed point with SO(5)
symmetry. We discuss the possibility that this fixed point describes a
transition between the Neel and valence bond solid states, and the relationship
to the SO(5) non-linear sigma model of Tanaka and Hu.
