Spinning "Snowballs" in SuperfluidHe4

We propose a simple mechanism that generates nonparabolicity in the dispersion of the positive charge carrier (snowball) in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. 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 that we calculate leads to a linear temperature variation of the snowball effective mass. The model is consistent with recent measurements if we assume a mass anisotropy of (9-13)%. An additional $T^3$ mass enhancement by long-wavelength phonons is negligible for the snowball but may be observable for the negative charge carrier.