  We have carried out a study of the momentum distribution and of the spectrum
of elementary excitations of liquid $^4$He across the normal-superfluid
transition temperature, using the path integral Monte Carlo method. Our results
for the momentum distribution in the superfluid regime show that a kink is
present in the range of momenta corresponding to the roton excitation. This
effect disappears when crossing the transition temperature to the normal fluid,
in a behavior currently unexplained by theory.
