Non-equilibrium effective field theory and second sound

A bstract We investigate the phenomenon of second sound in various states matter from perspective non-equilibrium effective field theory (EFT). In particular, for each state considered, we find that at least two (though sometimes multiple) qualitatively different EFTs exist finite temperature such there is always one EFT with a propagating second-sound wave and no wave. To aid construction these EFTs, use method cosets developed systems. It turns out difference between without modes can be understood as arising choices new kind inverse Higgs constraint. Finally, demonstrate it possible to bypass need constraints by formulating on manifold like usual fluid worldvolume, but reduced gauge symmetries.