Quantum and classical behavior of single-particle dynamics in dense liquidHe4

Inelastic-neutron-scattering measurements at high exchanged momentum have been performed in fluid $^{4}\mathrm{He}$ along a supercritical isochore in the 4.2--50 K temperature range. The data have been analyzed in the plane-wave impulse approximation to obtain the root-mean-square values of the particle momentum. Deviations from classical behavior have been interpreted as arising from the quantum nature of the particle oscillations in the framework of a cell model for the fluid.