Gapless nonhydrodynamic modes in relativistic kinetic theory
Lorenzo Gavassino
Abstract
We provide rigorous criteria to determine whether the nonhydrodynamic sector of a relativistic kinetic theory is gapless. These general criteria apply to both Boltzmann's equation and approximate models thereof, provided that the latter are consistent with thermodynamic principles. As an application of these criteria, we prove that the nonhydrodynamic sector is gapless whenever the scattering cross section decays to zero at large energies. Physically, this happens because, if particles with very high energy (compared to the temperature) are free streaming, we can use them to build hot nonhydrodynamic waves, which live longer than any hydrodynamic wave. Since many standard cross sections in quantum field theory vanish at high energies, the existence of these nonthermal long-lived waves is a rather general feature of relativistic systems. Published by the American Physical Society 2024