The dynamic structure factor of superfluid 4He has been investigated at very low temperatures by inelastic
neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large
dynamic range with excellent energy resolution, covering wave vectors Q up to 5 A⁻¹ and energies ω up to
15 meV. A detailed description of the dynamics of superfluid 4He is obtained from saturated vapor pressure
up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon
energy exceeds the roton-roton decay threshold. A highly structured multiexcitation spectrum is observed at low
energies,where clear thresholds and branches have been identified. Strong phonon emission branches are observed
when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong
multiexcitations whenever the single excitations face disintegration following Pitaevskii’s type a or b criteria.
At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the
recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the
dynamic many-body calculations, even at the highest densities, where the correlations are strongest.