Deep Inelastic Neutron Scattering from Liquid HELIUM-4

We have performed an extensive series of high resolution deep inelastic neutron scattering measurements on liquid ^4He at a number of high momentum transfers for several temperatures and densities in both the normal and superfluid phases. The primary goal of these measurements was to obtain information on the momentum distribution of liquid ^4He. The shape of the momentum distribution of liquid ^4 He in the superfluid phase is of fundamental interest since, according to our present theoretical understanding of superfluidity in bulk ^4He, a delta-function singularity at p = 0 should exist due to the Bose condensate. A secondary goal, motivated by our momentum distribution studies, was to investigate the nature of the corrections to the Impulse Approximation, called final state effects (FSE), in deep inelastic neutron scattering from liquid ^4 He. The observed Compton profiles, whose shape depends on both the underlying momentum distribution and FSE, have been analyzed under two complementary assumptions. Under the assumption that calculations of the momentum distribution using the Green's Function Monte Carlo method are accurate, we have used the low density scattering data to evaluate FSE theories and to extract the form of FSE corrections. At a momentum transfer of Q = 23A^{-1 }, we find that only recent theories due to Silver and Carraro and Koonin are consistent with the data. At lower values of Q, we find that no current theory describes the observed FSE completely. Under the assumption that the recent theory by Silver for FSE in liquid ^4He is accurate, we have compared the data at Q = 23A ^{-1} with theoretical calculations of the momentum distribution. We obtain excellent agreement between the theoretical calculations of the momentum distribution, Silver's theory for FSE, and the scattering data at all temperatures and densities in both phases. In addition, we have extracted information on the shape of the underlying momentum distribution, including the average kinetic energy and the condensate fraction. Our results provide strong evidence for a narrow component in the momentum distribution of the superfluid phase corresponding to the Bose condensate.