DEVIATIONS FROM THE IMPULSE APPROXIMATION IN LIQUID-HE-4 - AN EXPERIMENTAL TEST AT Q=23A-1

Deep-inelastic-scattering measurements at high-momentum transfer Q can provide direct information on the momentum distribution of a system of particles if the impulse approximation (IA) is valid. In many such experiments, however, deviations between the observed scattering and predictions based on the IA are present. In liquid He-4 these deviations, called final-state effects (FSE), are caused by interactions among the particles. We have used liquid He-4 as a testing ground for the study of FSE. Deep-inelastic-neutron-scattering measurements on liquid He-4 have been carried out for temperatures of 0.35 and 3.5 K at a density of 0.147 g/cm3. Under the assumption that current theoretical calculations of the momentum distribution of liquid He-4 are accurate, we extract the form of FSE in the superfluid phase from the scattering data. We also compare the predictions of several theories for FSE to the experimental data. At the momentum transfer of Q = 23 angstrom-1 reached in these measurements, we find that an FSE theory due to Silver is the only current theory in agreement with the data in both the normal and superfluid phases.