Highly out-of-equilibrium 1D gases: experiments and generalized hydrodynamics *

Non-equilibrium dynamics in nearly integrable systems, long a theoretical challenge in most regimes, can now be calculated using generalized hydrodynamics (GHD). GHD depends on the continuum approximation and the assumption that the generalized Gibbs ensemble is always locally satisfied. We have tested GHD using ensembles of 1D Bose gases, challenging its two central assumptions in experiments with as few as a dozen particles per gas and using very strong trap quenches, after which the density increases by as much as 35 times. We find excellent theory-experiment agreement for a range of dimensionless coupling strengths and for long times. We follow these dynamics by directly measuring the evolving distribution of rapidities (the momenta of the systems quasiparticles). By also measuring evolving momentum distributions, we can effectively observe how the nature of the quasiparticles themselves change.