  We compute the dynamics of localized excitations produced by a quantum quench
in the spin 1/2 XXZ chain. Using numerics combining the density matrix
renormalization group and exact time evolution, as well as analytical
arguments, we show that fractionalization due to interactions in the pre-quench
state gives rise to "ultrarelativistic" density waves that travel at the
maximum band velocity. The system is initially prepared in the ground state of
the chain within the gapless XY phase, which admits a Luttinger liquid (LL)
description at low energies and long wavelengths. The Hamiltonian is then
suddenly quenched to a band insulator, after which the chain evolves unitarily.
Through the gapped dispersion of the insulator spectrum, the post-quench
dynamics serve as a "velocity microscope," revealing initial state particle
correlations via space time density propagation. We show that the
ultrarelativistic wave production is tied to the particular way in which
fractionalization evades Pauli-blocking in the zero-temperature initial LL
state.
