High-resolution measurements of rotons in He-4

A direct measurement of the roton line shape in superfluid He-4 down to T = 0.8 K has been possible using neutron backscattering with a measured energy resolution of less than 1 mu eV. These results constitute a significant improvement on the previous best determination and extend the measured range of roton linewidth and energy shift by an order of magnitude. The temperature dependence of the measured linewidths is found to be in excellent agreement with calculations based on the 4-phonon decay process. The roton energy, however, is found to vary significantly more slowly with temperature than expected from the same theory. The origin of this discrepancy is not yet understood.