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.

Figure 1

Antiprotons are loaded from below (left), into the trap electrodes below the rotatable electrode. Positrons are simultaneously loaded from above (right) into the electrodes above the rotatable electrode.

Figure 2

Cross section of trap electrode (a), potential on axis (b) and the magnitude of the axial electric field on axis (c) and 3 mm off axis (d). Solid curves are the static potentials and axial electric fields magnitudes. Dashed and dotted curves show the maximum variation of these when the oscillating potential is added.

Figure 3

The fraction of the $\overline{\mathrm{H}}$ atoms detected in the detection well decreases as the frequency $\omega /(2\pi )$ of the oscillating electric field is increased (solid points). More atoms are detected when there is no oscillating field (open square). This point is plotted on the same scale as the others but it has no frequency associated with it. The measured points are compared to a simple model discussed in the text; the solid curves apply when the oscillating electric field is applied, and the dashed curve when it is not.