Dynamics of the O (4) critical point in QCD

We perform a Langevin simulation of the O(4) critical point, which lies in the dynamic universality class of "model G."This is the dynamic universality class of the chiral phase transition in QCD with two massless flavors. The axial charge and the order parameter φa=(σ,π→) exhibit a rich dynamical interplay, which reflects the qualitative differences in the hydrodynamic effective theories above and below Tc. From the axial charge correlators on the critical line we extract a dynamical critical exponent of ζ=1.47±0.01(stat.), which is compatible with the theoretical expectation of ζ=d/2 (with d=3) when systematic errors are taken into account. At low temperatures, we quantitatively match the O(4) simulations to the superfluid effective theory of soft pions.