  High-resolution neutron resonance spin-echo measurements of superfluid 4He
show that the roton energy does not have the same temperature dependence as the
inverse lifetime. Diagrammatic analysis attributes this to the interaction of
rotons with thermally excited phonons via both four- and three-particle
processes, the latter being allowed by the broken gauge symmetry of the Bose
condensate. The distinct temperature dependence of the roton energy at low
temperatures suggests that the net roton-phonon interaction is repulsive.
