The nature of the short wavelength excitations in vitreous silica:   X-Rays Brillouin scattering study
  The dynamical structure factor (S(Q,E)) of vitreous silica has been measured by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed exchanged energy (E) - an experimental procedure that, contrary to the usual one at constant Q, provides spectra with much better identified inelastic features. This allows the first direct evidence of Brillouin peaks in the S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that excludes the possibility that the BP marks the transition from propagating to localised dynamics in glasses. 
O.Pilla, A.Cunsolo, A.Fontana, C.Masciovecchio, G.Monaco, M.Montagna,   G.Ruocco, T.Scopigno, and F.Sette