Direct Evidence for R − Rotons Having Antiparallel Momentum and Velocity

Abstract

Experimental evidence is presented that shows that the momentum of a R ⁻ roton (a particle-like excitation in liquid helium-4) is antiparallel to its velocity. Although this is anticipated from the negative slope of the dispersion curve for these excitations, it has only been possible to test since the development of a source of ballistic R ⁻ rotons. The backward refraction of the quantum evaporation process, which is the signature of antiparallel momentum and velocity, is observed.