Evidence for Roton Pair Creation in Superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$

We have measured the drift velocity $\overline{v}$ of negative ions through superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ for temperatures $T<\mathrm{}0.45\mathrm{}$ K, under a pressure of 25 bar, for electric fields $20<E<\mathrm{}2.6\mathrm{}\times {10}^4$ V ${\mathrm{cm}}^{-1\mathrm{}}$. From the observed form of $\overline{v}\mathrm{}\left(E\right)\mathrm{}$, 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.