Neutron scattering as a probe of particle-like excitations in superfluid4He: A model calculation

In the microscopic theory of Bose-condensed liquids, the condensate wave function leads to a coupling of the density fluctuations and the single-particle excitations. A simple parametrized model of this based on coupling weakly damped zero-sound modes with strongly damped free-particle-like excitations similar to Q/sup 2//2m explains why the phonon-roton resonance in S(Q,w) for superfluid /sup 4/He exists above and below Tlambda but only pays the role of an elementary excitation below Tlambda. It is pointed out that this model also predicts that S(Q,w) should exhibit a weak, low-frequency peak centered at Q/sup 2//2m, with weight roughly proportional to the condensate fraction n/OMICRON/. As a stimulus to further experimental searches for this branch, some model calculations are given for S(Q,w) for Q similar to 1A/sup -1/.