Title: The Momentum Distribution of Liquid $^4$He,
Abstract: We report high-resolution neutron Compton scattering measurements of liquid
$^4$He under saturated vapor pressure. There is excellent agreement between the
observed scattering and ab initio predictions of its lineshape. Quantum Monte
Carlo calculations predict that the Bose condensate fraction is zero in the
normal fluid, builds up rapidly just below the superfluid transition
temperature, and reaches a value of approximately $7.5\%$ below 1 K. We also
used model fit functions to obtain from the scattering data empirical estimates
for the average atomic kinetic energy and Bose condensate fraction. These
quantities are also in excellent agreement with ab initio calculations. The
convergence between the scattering data and Quantum Monte Carlo calculations is
strong evidence for a Bose broken symmetry in superfluid $^4$He.