First Measurement of the Velocity of Slow Antihydrogen Atoms

The speed of antihydrogen atoms is deduced from the fraction that passes through an oscillating electric field without ionizing. The weakly bound atoms used for this first demonstration travel about $20$ times more rapidly than the average thermal speed of the antiprotons from which they form, if these are in thermal equilibrium with their 4.2 K container. The method should be applicable to much more deeply bound states, which may well be moving more slowly, and should aid the quest to lower the speed of the atoms as required if they are to be trapped for precise spectroscopy.