Phantom energy in the nonlinear response of a quantum many-body scar state

Quantum many-body scars are notable as nonthermal, low-entanglement states that exist at high energies. We used attractively interacting one-dimensional (1D) dysprosium gases to create scar states that are stable enough to be driven into a strongly nonlinear regime while retaining their character. We uncovered an emergent nonlinear many-body phenomenon, the effective transmutation of attractive interactions into repulsive interactions. We measured how the kinetic and total energies evolve after quenching the confining potential. Although the bare interactions are attractive, the atoms behaved as if they repel each other: Their kinetic energy paradoxically decreased as the gas is compressed. The missing "phantom" energy is quantified by benchmarking our experimental results against generalized hydrodynamics calculations. We present evidence that the missing kinetic energy is carried by undetected, very high-momentum atoms.