Hydrodynamic relaxation of spin helices

Motivated by recent cold-atom experiments, we study the relaxation of spin helices in quantum XXZ chains. The experimentally observed follows scaling laws that are qualitatively different from linear-response transport. We construct a theory such helices, combining generalized hydrodynamics with diffusive corrections and form local density approximation. Although far equilibrium, our hydrodynamic approach reproduces relaxational dynamics also predicts late-time relaxation, which is outside accessible timescales. In particular, explains existence temporal regimes apparent anomalous diffusion, as well asymmetry between positive negative anisotropy at short intermediate times.