Breakdown of the Gaussian approximation in semi-quantum liquids

We have measured the incoherent scattering function of liquid para-hydrogen, pure and mixed with neon, in various thermodynamic conditions, using inelastic neutron scattering. The experiments were carried out on TOSCA-II, a time-of-flight, inverse-geometry, crystal-analyzer spectrometer. After an accurate data reduction, the low-energy parts of neutron spectra recorded in forward scattering have been analyzed in the framework of the Gaussian approximation and fitted through a Levesque-Verlet model for the velocity auto-correlation function. Despite the excellent quality of these fits, the velocity auto-correlation function derived from the forward-scattering data appears totally unable to properly describe the backward-scattering ones. These findings prove a clear breakdown of the Gaussian approximation in these semi-quantum liquids.