Spinning snowballs in superfluid sup 4 He.

We propose a simple mechanism that generates nonparabolicity in the dispersion of the positive charge carrier (snowball) in superfluid sup 4 He. Specifically, we pursue the consequences of a model where (1) the snowball behaves as a rigid rotator and (2) the hydrodynamic contribution to its mass (with rotator axes fixed) is anisotropic. The nonparabolicity we calculate leads to a linear temperature variation of the snowball effective mass. The model is consistent with recent measurements assuming a mass anisotropy of 9-13%. An additional T sup 3 mass enhancement by long wavelength phonons is negligible for the snowball but may be observable for the negative charge carrier.