The dynamic structure factor of 4He beyond the roton minimum

The dynamic structure factor S((q) over right arrow ,w) of He-4 is studied at zero temperature in the momentum region at and above the roton minimum by Field-theoretical methods. The model is derived from the Gavoret-Nozieres (GN) two particle propagator by introducing the concept of quasiparticles. In this way a connection between the field theory of GN and the phenomenological models of Zawadowski-Ruvalds-Solana (ZRS) type is obtained. An, improved expression for the dynamic structure factor within ZRS-like models is found. Numerical results for S((q) over right arrow ,w) are presented for momentum and energy independent effective interactions between quasiparticles.