Evidence for Upward or "Anomalous" Dispersion in the Excitation Spectrum of He II

The dispersion and attenuation of superthermal phonons ($h\omega \gg \mathrm{kT}$) in He II at 0.1 K are remeasured and reinterpreted as a function of pressure. We find evidence for the existence of the three-phonon process and an upper energy cutoff to this process implying anomalous dispersion. Our earlier conclusion on the absence of anomalous dispersion at saturated vapor pressure is shown to be incorrect.