  The O(4) linear sigma model of the chiral transition in QCD is similar to
models of the superfluid transition in 4He and 3He. Observations of vortex
formation in superfluid helium have recently improved the understanding of the
dynamics of such transitions. This is exploited to estimate the baryon density
in the central region of a heavy ion region and the result is consistent with
the long held belief that this density is very small in comparison with the
pion density.
