Evidence for Roton Pair Creation in Superfluid He4

We have measured the drift velocity $\overline{v}$ of negative ions through superfluid $^{4}\mathrm{He}$ for temperatures $T<0.45$ K, under a pressure of 25 bar, for electric fields $20<E<2.6\ifmmode\times\else\texttimes\fi{}{10}^{4}$ V ${\mathrm{cm}}^{\ensuremath{-}1}$. From the observed form of $\overline{v}(E)$, we deduce that roton emission from an ion traveling faster than the Landau critical velocity is governed by hitherto unrecognized selection rules whereby the rotons can be created only in pairs.