Deviations from the impulse approximation in liquid 4He: Momentum-transfer dependence.

Abstract

We have carried out a series of deep-inelastic-neutron-scattering (DINS) measurements on liquid $^{4}\mathrm{He}$ to study the Q dependence of deviations from the impulse approximation, called final-state effects (FSE). DINS measurements on liquid $^{4}\mathrm{He}$ have been carried out for temperatures of 0.75 and 4.25 K at a density of 0.147 g/${\mathrm{cm}}^{3}$ for momentum transfers between 10 and 23 \AA{} $^{\mathrm{\ensuremath{-}}1}$. Under the assumption that current theoretical calculations of the momentum distribution of liquid $^{4}\mathrm{He}$ are accurate, we extract the form of FSE in the superfluid phase from the scattering data. We find that FSE are definitely present in the scattering data, and their effects are more important at lower Q's. We also compare the predictions of several theories for FSE to the experimental data. We find that the recent theories of FSE of Silver and of Carraro and Koonin are in excellent agreement with the data in the superfluid phase at Q=23 \AA{} $^{\mathrm{\ensuremath{-}}1}$. At the lower values of Q, deviations from these theories appear. No current theory describes the observed FSE completely at the lower Q's.