Evidence for a New Kind of Energetic Neutral Excitation in Superfluid Helium

We present evidence showing that neutral excitations which are not photons are produced in superfluid liquid helium in the presence of a ${\mathrm{Po}}^{210}$ alpha-particle source. At low temperatures these excitations travel through the liquid with negligible scattering. They have sufficient energy to generate positive ions and electrons at the surface of the liquid or at a suitable metal plate immersed in the liquid.