The Internal States of Anti-Hydrogen

The internal states of anti-hydrogen have been studied, for the first time, by the ATRAP collaboration, working at CERN where antiprotons are slowed and then joined with positrons to form anti-hydrogen atoms within a detector- and electrode-filled enclosure (a nested Penning trap) over the past year. This work suggests that anti-hydrogen is preferentially formed in an excited state via a three-body process when two positrons and an anti-proton collide.

Only a month ago the ATHENA collaboration, working at the same CERN facility, made the first report of cold anti-H atom detection (Update 605) using techniques largely pioneered by ATRAP (the antiproton accumulation techniques, the nested Penning trap used, the positron cooling approach, etc.) So what has changed since then? Three things:

(1) First of all, the ATHENA detection of anti-atoms is indirect. The presumed presence of the anti-atom (positron plus antiproton) is registered by a dual annihilation of the positron with an electron and the antiproton with a nearby proton.

Complicating the detection scenario is the fact that the proton-antiproton annihilation itself sometimes spawns positrons which (when they annihilate in their turn) could falsely indicate the prior presence of an anti-atom.

This class of events constitutes a background which must be subtracted out in the analysis process, and it precludes one from identifying any particular double-annihilation event as having been a genuine anti-hydrogen (sometimes written as an H with a bar over it).

By contrast, the ATRAP direct detection process unambiguously identifies H-bar in a process called field ionization, which works as follows. Having formed in the center of the enclosure, neutral anti-atoms are free to drift in any direction.

Some of them annihilate but others move into an "ionization well," a region where strong electric fields tear the H-bar apart. Negatively charged antiprotons not in the company of a positively charged positron cannot reach the well.

Once there, though, the field sunders the atom, and the antiprotons are trapped in place, leaving the positron to move off and annihilate elsewhere. By counting the number of antiprotons one knows how many anti-atoms had arrived at the well. Every event represents an anti-atom. (See figure.)

(2) Moreover, one can now make a statistical study of the electric field needed to ionize the positron and deduce from this, in a rudimentary way, some information about the internal energy states of the H-bar. Thus the internal properties of an anti-atom have been studied for the first time. The observed range in principal quantum number n (n=1 corresponding to the ground state, or lowest level) goes from 43 up to 55.

(3) Finally, another thing that is different in this experiment is the much higher rate of anti-H production. The collaboration spokesperson, Gerald Gabrielse of Harvard (617-495-4381, CERN 41-22-767-9813, gabrielse@physics.harvard.edu) says that more anti-H atoms can be recorded in a few hours than have been reported in all previous experiments.

The ultimate goal of these experiments will be to trap neutral cold anti-hydrogen atoms and to study their spectra with the same precision (parts per 10¹⁴ for an analysis of the transition from the n=2 to the n=1 state) as for plain hydrogen. One could then tell whether the laws of physics apply the same or differently to atoms and anti-atoms. (Gabrielse et al., Physical Review Letters, 18 November; other ATRAP contacts are Walter Oelert at Forschungszentrum Julich, 49-2461-61-4156, CERN 41-22-767-1758; Jochen Walz at the Max Planck Institute for Quantum Physics, 49-89-32905-281, CERN 41-22-767-9813; Eric Hessels at York University, CERN 41-22-767-9813; ATRAP website).

In recent work ATRAP sees a further increase in the antihydrogen production rate by using a small radio transmitter to heat antiprotons into making repeated collisions with cold positrons. With this higher production rate, they are able to make the first measurements of a distribution (not just the range) of excited states of antihydrogen. (For an early background article, by Gabrielse, see Scientific American, Dec 1992.)