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Number 611, October 29, 2002 by Phil Schewe, James Riordon, and Ben Stein

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 , 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.)

Testing New Physics With Nothing

To detect new forces, particles, and dimensions in a sub-micron-sized force experiment, physicists must inevitably confront the Casimir force, an exotic quantum phenomenon in which empty space can push together a pair of metal plates. Empty space, or the "vacuum," is actually teeming with fleeting particles and electromagnetic fields.

But in between a pair of narrowly spaced plates, the vacuum does not pack energy as densely as it does outside the plates. Just as an underground tunnel blocks AM radio signals with wavelengths that are bigger than the opening of the tunnel, the metal plates keep out electromagnetic fluctuations with wavelengths greater than the distance between the plates.

In a 1600s demonstration by Otto von Guericke, the invisible atmosphere pushed together a pair of evacuated hemispheres so strongly that even horses could not pull them apart. Similarly, in the Casimir effect, the more energy-dense vacuum outside the plates pushes together the metal plates, because they enclose a less energy-dense vacuum.

However, the Casimir effect occurs much more subtly than the 1600s demonstration. Nonethless, this vacuum pressure, which has been confirmed experimentally (Update 300), can become large enough at short separations to conceal the effects of new physics.

To overcome this problem, theorists at Purdue University and Wabash College (contact Dennis Krause, kraused@wabash.edu) propose to exploit a key fact: the metal material itself influences the strength of the Casimir force, primarily through electronic interactions between the metal and the vacuum.

On the other hand, the plates' interaction with any new forces, particles, or dimensions would likely depend on the metal's nuclear as well as electronic properties.

Therefore, the theorists suggest making differential measurements of the Casimir force. Together with experimentalists at IUPUI and Lucent, they would compare the Casimir force for plates made with different metal isotopes of the same element. Isotopes of an element have fairly identical electronic properties, but different nuclear and gravitational properties.

If there is a difference between the measured Casimir forces, the researchers can attribute it to new physics after other effects (such as sample preparation) are taken into account. (Isotopes do affect the plates' electronic properties slightly, but the researchers compute the resulting change in Casimir force to be tiny compared to other effects, about 10,000 times smaller than the magnitude of the Casimir force itself.)

Their technique has another advantage: by directly measuring Casimir force differences, rather than the force itself, they reduce the dependence upon theoretical assumptions. (Krause and Fischbach, Physical Review Letters, 4 November 2002; also Fischbach, Krause, Decca, Lopez, Physics Letters A, upcoming; also see Purdue News release).

Tooth and Nail

The architecture of many living creatures combines soft organic tissue with hard inorganic crystal. How do the hard parts develop while up against the soft parts?

To examine this issue, physicists at Northwestern University have grown an inorganic lattice (barium fluoride, BaF₂) directly beneath a two-dimensional crystalline array of organic molecules (a fatty acid).

Using the diffraction of synchrotron radiation from these planar arrays, the researchers observe the structure of the two lattices and also affirm that the two become commensurate (that is, they register with each other), the first time this has been done in an experiment.

Even though the lattice spacings of the BaF₂ and the organic monolayer are different, each contributes toward a compromise, the barium fluoride structure by contracting just a bit, and the molecules by expanding their spacing at one end: picture the molecules as a stack of pencils standing on end and then being tilted a bit, modifying the spacing of the pencil tips (see figure).

BaF₂ is not a biologically important mineral, but the Northwestern scientists (contact Pulak Dutta, 847-491-5465, pdutta@northwestern.edu) expect to look directly at biomineralization in an upcoming phase of their work.

Furthermore, since growing two or more incommensurate materials next to each other (an important operation in the microelectronics industry) is difficult because of the unequal atomic spacings, the new research might in the long run be able to lessen or end the currently stringent need for high vacuum to make epitaxially grown materials. (Kmetko et al., Physical Review Letters, 28 Oct)