Supercooled Goldstone Bosons at the QCD Chiral Phase Transition

We discuss a universal non-equilibrium enhancement of long-wavelength Goldstone bosons induced by quenches to the broken phase in Model G -- the dynamical universality class of an $O(4)$-antiferromagnet and the chiral phase transition in QCD. Scaling arguments for the coarsening dynamics describing the formation of the chiral condensate predict a parametric enhancement in the infrared spectra of Goldstones, a prediction confirmed by stochastic simulations of the transition. The details of the enhancement are determined by the non-linear dynamics of a superfluid effective theory, which is a limit of Model G reflecting the broken $O(4)$ symmetry. Our results translate to a parametric enhancement of low-momentum pions in heavy-ion collisions at the LHC, which are underpredicted in current hydrodynamic models without critical dynamics.