Abstract
A novel experiment has been performed which gives direct evidence for the existence of long lived negative group velocity (R-) rotons in superfluid liquid helium at T = 0.1 K. Such excitations produced by the condensation of an atomic helium beam are found to propagate ballistically in the liquid, reflect specularly from a crystal surface and are able to evaporate atoms in a quantum evaporation process. The component of momentum parallel to the surface is conserved which shows that the momentum of the R- rotons is oppositely directed to the group velocity.
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